cmd/compile: improve tighten pass

cmd/compile: improve tighten pass

Move a value to the block which is the lowest common ancestor in the
dominator tree of all of its uses.  Make sure not to move a value into a
loop.

Makes the tighten pass on average (across go1 benchmarks) 40% slower.
Still not a big contributor to overall compile time.

Binary size is just a tad smaller.

name                      old time/op    new time/op    delta
BinaryTree17-12              2.77s ± 9%     2.76s ± 9%     ~     (p=0.878 n=8+8)
Fannkuch11-12                2.75s ± 1%     2.74s ± 1%     ~     (p=0.232 n=8+7)
FmtFprintfEmpty-12          48.9ns ± 9%    47.7ns ± 0%     ~     (p=0.431 n=8+8)
FmtFprintfString-12          143ns ± 8%     142ns ± 1%     ~     (p=0.257 n=8+7)
FmtFprintfInt-12             123ns ± 1%     122ns ± 1%   -1.04%  (p=0.026 n=7+8)
FmtFprintfIntInt-12          195ns ± 7%     185ns ± 0%   -5.32%  (p=0.000 n=8+8)
FmtFprintfPrefixedInt-12     194ns ± 4%     195ns ± 0%   +0.81%  (p=0.015 n=7+7)
FmtFprintfFloat-12           267ns ± 0%     268ns ± 0%   +0.37%  (p=0.001 n=7+6)
FmtManyArgs-12               800ns ± 0%     762ns ± 1%   -4.78%  (p=0.000 n=8+8)
GobDecode-12                7.67ms ± 2%    7.60ms ± 2%     ~     (p=0.234 n=8+8)
GobEncode-12                6.55ms ± 0%    6.57ms ± 1%     ~     (p=0.336 n=7+8)
Gzip-12                      237ms ± 0%     238ms ± 0%   +0.40%  (p=0.017 n=7+7)
Gunzip-12                   40.8ms ± 0%    40.2ms ± 0%   -1.52%  (p=0.000 n=7+8)
HTTPClientServer-12          208µs ± 3%     209µs ± 3%     ~     (p=0.955 n=8+7)
JSONEncode-12               16.2ms ± 1%    17.2ms ±11%   +5.80%  (p=0.001 n=7+8)
JSONDecode-12               57.3ms ±12%    55.5ms ± 3%     ~     (p=0.867 n=8+7)
Mandelbrot200-12            4.68ms ± 6%    4.46ms ± 1%     ~     (p=0.442 n=8+8)
GoParse-12                  4.27ms ±44%    3.42ms ± 1%  -19.95%  (p=0.005 n=8+8)
RegexpMatchEasy0_32-12      75.1ns ± 0%    75.8ns ± 1%   +0.99%  (p=0.002 n=7+7)
RegexpMatchEasy0_1K-12       963ns ± 0%    1021ns ± 6%   +5.98%  (p=0.001 n=7+7)
RegexpMatchEasy1_32-12      72.4ns ±11%    70.8ns ± 1%     ~     (p=0.368 n=8+8)
RegexpMatchEasy1_1K-12       394ns ± 1%     399ns ± 0%   +1.23%  (p=0.000 n=8+7)
RegexpMatchMedium_32-12      114ns ± 0%     115ns ± 1%   +0.63%  (p=0.021 n=7+7)
RegexpMatchMedium_1K-12     35.9µs ± 0%    37.6µs ± 1%   +4.72%  (p=0.000 n=7+8)
RegexpMatchHard_32-12       1.93µs ± 2%    1.91µs ± 0%   -0.91%  (p=0.001 n=7+7)
RegexpMatchHard_1K-12       60.2µs ± 3%    61.2µs ±10%     ~     (p=0.442 n=8+8)
Revcomp-12                   404ms ± 1%     406ms ± 1%     ~     (p=0.054 n=8+7)
Template-12                 64.6ms ± 1%    63.5ms ± 1%   -1.66%  (p=0.000 n=8+8)
TimeParse-12                 347ns ± 8%     309ns ± 0%  -11.13%  (p=0.000 n=8+7)
TimeFormat-12                343ns ± 4%     331ns ± 0%   -3.34%  (p=0.000 n=8+7)

Change-Id: Id6da1239ddd4d0cb074ff29cffb06302d1c6d08f
Reviewed-on: https://go-review.googlesource.com/28712
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>