cmd/compile: improve startRegs calculation

cmd/compile: improve startRegs calculation

In register allocation, we calculate what values are used in
and after the current block. If a value is used only after a
function call, since registers are clobbered in call, we don't
need to mark the value live at the entrance of the block.
Before this CL it is considered live, and unnecessary copy or
load may be generated when resolving merge edge.

Fixes #14761.

On AMD64:
name                      old time/op    new time/op    delta
BinaryTree17-12              2.84s ± 1%     2.81s ± 1%   -1.06%  (p=0.000 n=10+9)
Fannkuch11-12                3.61s ± 0%     3.55s ± 1%   -1.77%  (p=0.000 n=10+9)
FmtFprintfEmpty-12          50.4ns ± 4%    50.0ns ± 1%     ~     (p=0.785 n=9+8)
FmtFprintfString-12         80.0ns ± 3%    78.2ns ± 3%   -2.35%  (p=0.004 n=10+9)
FmtFprintfInt-12            81.3ns ± 4%    81.8ns ± 2%     ~     (p=0.159 n=10+10)
FmtFprintfIntInt-12          120ns ± 4%     118ns ± 2%     ~     (p=0.218 n=10+10)
FmtFprintfPrefixedInt-12     152ns ± 3%     155ns ± 2%   +2.11%  (p=0.026 n=10+10)
FmtFprintfFloat-12           240ns ± 1%     238ns ± 1%   -0.79%  (p=0.005 n=9+9)
FmtManyArgs-12               504ns ± 1%     510ns ± 1%   +1.14%  (p=0.000 n=8+9)
GobDecode-12                7.00ms ± 1%    6.99ms ± 0%     ~     (p=0.497 n=9+10)
GobEncode-12                5.47ms ± 1%    5.48ms ± 1%     ~     (p=0.218 n=10+10)
Gzip-12                      258ms ± 2%     256ms ± 1%   -0.96%  (p=0.043 n=10+9)
Gunzip-12                   38.6ms ± 0%    38.3ms ± 0%   -0.64%  (p=0.000 n=9+8)
HTTPClientServer-12         90.4µs ± 3%    87.2µs ±11%     ~     (p=0.053 n=9+10)
JSONEncode-12               15.6ms ± 0%    15.6ms ± 1%     ~     (p=0.077 n=9+9)
JSONDecode-12               55.1ms ± 1%    54.6ms ± 1%   -0.85%  (p=0.010 n=10+9)
Mandelbrot200-12            4.49ms ± 0%    4.47ms ± 0%   -0.25%  (p=0.000 n=10+8)
GoParse-12                  3.38ms ± 0%    3.37ms ± 1%     ~     (p=0.315 n=8+10)
RegexpMatchEasy0_32-12      82.5ns ± 4%    82.0ns ± 0%     ~     (p=0.164 n=10+8)
RegexpMatchEasy0_1K-12       203ns ± 1%     202ns ± 1%   -0.85%  (p=0.000 n=9+10)
RegexpMatchEasy1_32-12      82.3ns ± 1%    81.1ns ± 0%   -1.39%  (p=0.000 n=10+8)
RegexpMatchEasy1_1K-12       357ns ± 1%     357ns ± 1%     ~     (p=0.697 n=8+9)
RegexpMatchMedium_32-12      125ns ± 2%     126ns ± 2%     ~     (p=0.197 n=10+10)
RegexpMatchMedium_1K-12     39.6µs ± 3%    39.6µs ± 1%     ~     (p=0.971 n=10+10)
RegexpMatchHard_32-12       1.99µs ± 2%    1.99µs ± 4%     ~     (p=0.891 n=10+9)
RegexpMatchHard_1K-12       60.1µs ± 3%    60.4µs ± 3%     ~     (p=0.684 n=10+10)
Revcomp-12                   531ms ± 6%     441ms ± 0%  -16.94%  (p=0.000 n=10+9)
Template-12                 58.9ms ± 1%    58.7ms ± 1%     ~     (p=0.315 n=10+10)
TimeParse-12                 319ns ± 1%     320ns ± 4%     ~     (p=0.215 n=9+9)
TimeFormat-12                345ns ± 0%     333ns ± 1%   -3.36%  (p=0.000 n=9+10)
[Geo mean]                  52.2µs         51.6µs        -1.13%

On ARM64:
name                     old time/op    new time/op    delta
BinaryTree17-8              8.53s ± 0%     8.36s ± 0%   -1.89%  (p=0.000 n=10+10)
Fannkuch11-8                6.15s ± 0%     6.10s ± 0%   -0.67%  (p=0.000 n=10+10)
FmtFprintfEmpty-8           117ns ± 0%     117ns ± 0%     ~     (all equal)
FmtFprintfString-8          192ns ± 0%     192ns ± 0%     ~     (all equal)
FmtFprintfInt-8             198ns ± 0%     198ns ± 0%     ~     (p=0.211 n=10+10)
FmtFprintfIntInt-8          289ns ± 0%     291ns ± 0%   +0.59%  (p=0.000 n=7+10)
FmtFprintfPrefixedInt-8     320ns ± 2%     317ns ± 0%     ~     (p=0.431 n=10+8)
FmtFprintfFloat-8           538ns ± 0%     538ns ± 0%     ~     (all equal)
FmtManyArgs-8              1.17µs ± 1%    1.18µs ± 1%     ~     (p=0.063 n=10+10)
GobDecode-8                17.0ms ± 1%    17.2ms ± 1%   +0.83%  (p=0.000 n=10+10)
GobEncode-8                14.2ms ± 0%    14.1ms ± 1%   -0.78%  (p=0.001 n=9+10)
Gzip-8                      806ms ± 0%     797ms ± 0%   -1.12%  (p=0.000 n=6+9)
Gunzip-8                    131ms ± 0%     130ms ± 0%   -0.51%  (p=0.000 n=10+9)
HTTPClientServer-8          206µs ± 9%     212µs ± 2%     ~     (p=0.829 n=10+8)
JSONEncode-8               40.1ms ± 0%    40.1ms ± 0%     ~     (p=0.136 n=9+9)
JSONDecode-8                157ms ± 0%     151ms ± 0%   -3.32%  (p=0.000 n=9+9)
Mandelbrot200-8            10.1ms ± 0%    10.1ms ± 0%   -0.05%  (p=0.000 n=9+8)
GoParse-8                  8.43ms ± 0%    8.43ms ± 0%     ~     (p=0.912 n=10+10)
RegexpMatchEasy0_32-8       228ns ± 1%     227ns ± 0%   -0.26%  (p=0.026 n=10+9)
RegexpMatchEasy0_1K-8      1.92µs ± 0%    1.63µs ± 0%  -15.18%  (p=0.001 n=7+7)
RegexpMatchEasy1_32-8       258ns ± 1%     250ns ± 0%   -2.83%  (p=0.000 n=10+10)
RegexpMatchEasy1_1K-8      2.39µs ± 0%    2.13µs ± 0%  -10.94%  (p=0.000 n=9+9)
RegexpMatchMedium_32-8      352ns ± 0%     351ns ± 0%   -0.29%  (p=0.004 n=9+10)
RegexpMatchMedium_1K-8      104µs ± 0%     105µs ± 0%   +0.58%  (p=0.000 n=8+9)
RegexpMatchHard_32-8       5.84µs ± 0%    5.82µs ± 0%   -0.27%  (p=0.000 n=9+10)
RegexpMatchHard_1K-8        177µs ± 0%     177µs ± 0%   -0.07%  (p=0.000 n=9+9)
Revcomp-8                   1.57s ± 1%     1.50s ± 1%   -4.60%  (p=0.000 n=9+10)
Template-8                  157ms ± 1%     153ms ± 1%   -2.28%  (p=0.000 n=10+9)
TimeParse-8                 779ns ± 1%     770ns ± 1%   -1.18%  (p=0.013 n=10+10)
TimeFormat-8                823ns ± 2%     826ns ± 1%     ~     (p=0.324 n=10+9)
[Geo mean]                  144µs          142µs        -1.45%

Reduce cmd/go text size by 0.5%.

Change-Id: I9288ff983c4a7cf03fc0cb35b9b1750828013117
Reviewed-on: https://go-review.googlesource.com/38457
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@golang.org>