cmd/compile: make a copy of Phi input if it is still live

cmd/compile: make a copy of Phi input if it is still live

Register of Phi input is allocated to the Phi. So if the Phi
input is still live after Phi, we may need to use a spill. In
this case, copy the Phi input to a spare register to avoid a
spill.

Originally targeted the code in issue #16187, and this CL
indeed removes the spill, but doesn't seem to help on benchmark
result. It may help in general, though.

On AMD64:
name                      old time/op    new time/op    delta
BinaryTree17-12              2.79s ± 1%     2.76s ± 0%  -1.33%  (p=0.000 n=10+10)
Fannkuch11-12                3.02s ± 0%     3.14s ± 0%  +3.99%  (p=0.000 n=10+10)
FmtFprintfEmpty-12          51.2ns ± 0%    51.4ns ± 3%    ~      (p=0.368 n=8+10)
FmtFprintfString-12          145ns ± 0%     144ns ± 0%  -0.69%    (p=0.000 n=6+9)
FmtFprintfInt-12             127ns ± 1%     124ns ± 1%  -2.79%   (p=0.000 n=10+9)
FmtFprintfIntInt-12          186ns ± 0%     184ns ± 0%  -1.34%   (p=0.000 n=10+9)
FmtFprintfPrefixedInt-12     196ns ± 0%     194ns ± 0%  -0.97%    (p=0.000 n=9+9)
FmtFprintfFloat-12           293ns ± 2%     287ns ± 0%  -2.00%   (p=0.000 n=10+9)
FmtManyArgs-12               847ns ± 1%     829ns ± 0%  -2.17%   (p=0.000 n=10+7)
GobDecode-12                7.17ms ± 0%    7.18ms ± 0%    ~     (p=0.123 n=10+10)
GobEncode-12                6.08ms ± 1%    6.08ms ± 0%    ~      (p=0.497 n=10+9)
Gzip-12                      277ms ± 1%     275ms ± 1%  -0.47%   (p=0.028 n=10+9)
Gunzip-12                   39.1ms ± 2%    38.2ms ± 1%  -2.20%   (p=0.000 n=10+9)
HTTPClientServer-12         90.9µs ± 4%    87.7µs ± 2%  -3.51%   (p=0.001 n=9+10)
JSONEncode-12               17.3ms ± 1%    16.5ms ± 0%  -5.02%    (p=0.000 n=9+9)
JSONDecode-12               54.6ms ± 1%    54.1ms ± 0%  -0.99%    (p=0.000 n=9+9)
Mandelbrot200-12            4.45ms ± 0%    4.45ms ± 0%  -0.02%    (p=0.006 n=8+9)
GoParse-12                  3.44ms ± 0%    3.48ms ± 1%  +0.95%  (p=0.000 n=10+10)
RegexpMatchEasy0_32-12      84.9ns ± 0%    85.0ns ± 0%    ~       (p=0.241 n=8+8)
RegexpMatchEasy0_1K-12       867ns ± 3%     915ns ±11%  +5.55%  (p=0.037 n=10+10)
RegexpMatchEasy1_32-12      82.7ns ± 5%    83.9ns ± 4%    ~      (p=0.161 n=9+10)
RegexpMatchEasy1_1K-12       361ns ± 1%     363ns ± 0%    ~      (p=0.098 n=10+8)
RegexpMatchMedium_32-12      126ns ± 0%     126ns ± 1%    ~      (p=0.549 n=8+10)
RegexpMatchMedium_1K-12     38.8µs ± 0%    39.1µs ± 0%  +0.67%    (p=0.000 n=9+8)
RegexpMatchHard_32-12       1.95µs ± 0%    1.96µs ± 0%  +0.43%    (p=0.000 n=9+9)
RegexpMatchHard_1K-12       59.0µs ± 0%    59.1µs ± 0%  +0.27%   (p=0.000 n=10+9)
Revcomp-12                   436ms ± 1%     431ms ± 1%  -1.19%  (p=0.005 n=10+10)
Template-12                 56.7ms ± 1%    57.1ms ± 1%  +0.71%   (p=0.001 n=10+9)
TimeParse-12                 312ns ± 0%     310ns ± 0%  -0.80%   (p=0.000 n=10+9)
TimeFormat-12                336ns ± 0%     332ns ± 0%  -1.19%    (p=0.000 n=8+7)
[Geo mean]                  59.2µs         58.9µs       -0.42%

On PPC64:
name                     old time/op    new time/op    delta
BinaryTree17-2              4.67s ± 2%     4.71s ± 1%    ~     (p=0.421 n=5+5)
Fannkuch11-2                3.92s ± 1%     3.94s ± 0%  +0.46%  (p=0.032 n=5+5)
FmtFprintfEmpty-2           122ns ± 0%     120ns ± 2%  -1.80%  (p=0.016 n=4+5)
FmtFprintfString-2          305ns ± 1%     299ns ± 1%  -1.84%  (p=0.008 n=5+5)
FmtFprintfInt-2             243ns ± 0%     241ns ± 1%  -0.66%  (p=0.016 n=4+5)
FmtFprintfIntInt-2          361ns ± 1%     356ns ± 1%  -1.49%  (p=0.016 n=5+5)
FmtFprintfPrefixedInt-2     355ns ± 1%     357ns ± 1%    ~     (p=0.333 n=5+5)
FmtFprintfFloat-2           502ns ± 2%     498ns ± 1%    ~     (p=0.151 n=5+5)
FmtManyArgs-2              1.55µs ± 2%    1.59µs ± 1%  +2.52%  (p=0.008 n=5+5)
GobDecode-2                13.0ms ± 1%    13.0ms ± 1%    ~     (p=0.841 n=5+5)
GobEncode-2                11.8ms ± 1%    11.8ms ± 1%    ~     (p=0.690 n=5+5)
Gzip-2                      499ms ± 1%     503ms ± 0%    ~     (p=0.421 n=5+5)
Gunzip-2                   86.5ms ± 0%    86.4ms ± 1%    ~     (p=0.841 n=5+5)
HTTPClientServer-2         68.2µs ± 2%    69.6µs ± 3%    ~     (p=0.151 n=5+5)
JSONEncode-2               39.0ms ± 1%    37.2ms ± 1%  -4.65%  (p=0.008 n=5+5)
JSONDecode-2                122ms ± 1%     126ms ± 1%  +2.63%  (p=0.008 n=5+5)
Mandelbrot200-2            6.08ms ± 1%    5.89ms ± 1%  -3.06%  (p=0.008 n=5+5)
GoParse-2                  5.95ms ± 2%    5.98ms ± 1%    ~     (p=0.421 n=5+5)
RegexpMatchEasy0_32-2       331ns ± 1%     328ns ± 1%    ~     (p=0.056 n=5+5)
RegexpMatchEasy0_1K-2      1.45µs ± 0%    1.47µs ± 0%  +1.13%  (p=0.008 n=5+5)
RegexpMatchEasy1_32-2       359ns ± 0%     353ns ± 0%  -1.84%  (p=0.008 n=5+5)
RegexpMatchEasy1_1K-2      1.79µs ± 0%    1.81µs ± 1%  +1.16%  (p=0.008 n=5+5)
RegexpMatchMedium_32-2      420ns ± 2%     413ns ± 0%  -1.72%  (p=0.008 n=5+5)
RegexpMatchMedium_1K-2     70.2µs ± 1%    69.5µs ± 1%  -1.09%  (p=0.032 n=5+5)
RegexpMatchHard_32-2       3.87µs ± 1%    3.65µs ± 0%  -5.86%  (p=0.008 n=5+5)
RegexpMatchHard_1K-2        111µs ± 0%     105µs ± 0%  -5.49%  (p=0.016 n=5+4)
Revcomp-2                   1.00s ± 1%     1.01s ± 2%    ~     (p=0.151 n=5+5)
Template-2                  113ms ± 1%     113ms ± 2%    ~     (p=0.841 n=5+5)
TimeParse-2                 555ns ± 0%     550ns ± 1%  -0.87%  (p=0.032 n=5+5)
TimeFormat-2                736ns ± 1%     704ns ± 1%  -4.35%  (p=0.008 n=5+5)
[Geo mean]                  120µs          119µs       -0.77%

Reduce "spilled value remains" by 0.6% in cmd/go on AMD64.

Change-Id: If655df343b0f30d1a49ab1ab644f10c698b96f3e
Reviewed-on: https://go-review.googlesource.com/32442
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>