cmd/compile/internal/ssa: optimize 386's subtraction

cmd/compile/internal/ssa: optimize 386's subtraction

The SUBL instruction can take a memory operand, and this CL
implements this optimization.

The go1 benchmark shows a little improvement.

name                     old time/op    new time/op    delta
BinaryTree17-4              3.27s ± 2%     3.29s ± 3%    ~     (p=0.322 n=37+40)
Fannkuch11-4                3.49s ± 0%     3.53s ± 1%  +1.21%  (p=0.000 n=31+40)
FmtFprintfEmpty-4          46.2ns ± 3%    46.3ns ± 2%    ~     (p=0.351 n=40+28)
FmtFprintfString-4         82.0ns ± 3%    81.5ns ± 2%  -0.69%  (p=0.002 n=40+30)
FmtFprintfInt-4            94.6ns ± 3%    94.6ns ± 6%    ~     (p=0.913 n=39+37)
FmtFprintfIntInt-4          147ns ± 3%     150ns ± 2%  +1.72%  (p=0.000 n=40+25)
FmtFprintfPrefixedInt-4     186ns ± 3%     186ns ± 0%  -0.33%  (p=0.006 n=40+25)
FmtFprintfFloat-4           388ns ± 4%     388ns ± 4%    ~     (p=0.162 n=40+40)
FmtManyArgs-4               612ns ± 3%     616ns ± 4%    ~     (p=0.223 n=40+40)
GobDecode-4                7.35ms ± 5%    7.42ms ± 5%    ~     (p=0.095 n=40+40)
GobEncode-4                7.21ms ± 8%    7.23ms ± 4%    ~     (p=0.294 n=40+40)
Gzip-4                      360ms ± 4%     359ms ± 4%    ~     (p=0.097 n=40+40)
Gunzip-4                   46.1ms ± 3%    45.6ms ± 3%  -1.20%  (p=0.000 n=40+40)
HTTPClientServer-4         64.0µs ± 2%    64.1µs ± 2%    ~     (p=0.648 n=39+40)
JSONEncode-4               21.9ms ± 4%    22.1ms ± 5%    ~     (p=0.086 n=40+40)
JSONDecode-4               67.9ms ± 4%    66.7ms ± 4%  -1.63%  (p=0.000 n=40+40)
Mandelbrot200-4            5.19ms ± 3%    5.17ms ± 3%    ~     (p=0.881 n=40+40)
GoParse-4                  3.34ms ± 3%    3.28ms ± 2%  -1.78%  (p=0.000 n=40+40)
RegexpMatchEasy0_32-4       101ns ± 5%      99ns ± 3%  -2.40%  (p=0.000 n=40+40)
RegexpMatchEasy0_1K-4       851ns ± 1%     848ns ± 3%  -0.36%  (p=0.004 n=33+40)
RegexpMatchEasy1_32-4       109ns ± 5%     105ns ± 3%  -3.53%  (p=0.000 n=39+40)
RegexpMatchEasy1_1K-4      1.03µs ± 4%    1.03µs ± 3%    ~     (p=0.638 n=40+38)
RegexpMatchMedium_32-4      131ns ± 5%     127ns ± 4%  -3.36%  (p=0.000 n=38+40)
RegexpMatchMedium_1K-4     43.4µs ± 4%    43.2µs ± 3%  -0.46%  (p=0.008 n=40+40)
RegexpMatchHard_32-4       2.21µs ± 4%    2.23µs ± 1%  +0.77%  (p=0.014 n=40+28)
RegexpMatchHard_1K-4       67.6µs ± 4%    67.7µs ± 3%  +0.11%  (p=0.016 n=40+40)
Revcomp-4                   1.86s ± 3%     1.77s ± 2%  -4.81%  (p=0.000 n=40+40)
Template-4                 71.7ms ± 3%    71.6ms ± 4%    ~     (p=0.200 n=40+40)
TimeParse-4                 436ns ± 4%     433ns ± 3%    ~     (p=0.358 n=40+40)
TimeFormat-4                413ns ± 4%     412ns ± 3%    ~     (p=0.415 n=40+40)
[Geo mean]                 63.9µs         63.6µs       -0.49%

name                     old speed      new speed      delta
GobDecode-4               105MB/s ± 5%   104MB/s ± 5%    ~     (p=0.096 n=40+40)
GobEncode-4               106MB/s ± 7%   106MB/s ± 3%    ~     (p=0.385 n=39+40)
Gzip-4                   54.0MB/s ± 4%  54.0MB/s ± 4%    ~     (p=0.100 n=40+40)
Gunzip-4                  421MB/s ± 3%   426MB/s ± 3%  +1.21%  (p=0.000 n=40+40)
JSONEncode-4             88.5MB/s ± 5%  88.0MB/s ± 5%    ~     (p=0.083 n=40+40)
JSONDecode-4             28.6MB/s ± 4%  29.1MB/s ± 4%  +1.65%  (p=0.000 n=40+40)
GoParse-4                17.3MB/s ± 3%  17.7MB/s ± 2%  +1.82%  (p=0.000 n=40+40)
RegexpMatchEasy0_32-4     316MB/s ± 5%   323MB/s ± 4%  +2.44%  (p=0.000 n=40+40)
RegexpMatchEasy0_1K-4    1.20GB/s ± 1%  1.21GB/s ± 3%  +0.40%  (p=0.004 n=33+40)
RegexpMatchEasy1_32-4     291MB/s ± 7%   302MB/s ± 4%  +3.82%  (p=0.000 n=40+40)
RegexpMatchEasy1_1K-4     993MB/s ± 4%   990MB/s ± 3%    ~     (p=0.623 n=40+38)
RegexpMatchMedium_32-4   7.61MB/s ± 5%  7.87MB/s ± 4%  +3.36%  (p=0.000 n=38+40)
RegexpMatchMedium_1K-4   23.6MB/s ± 4%  23.7MB/s ± 4%  +0.46%  (p=0.007 n=40+40)
RegexpMatchHard_32-4     14.5MB/s ± 4%  14.3MB/s ± 1%  -0.79%  (p=0.017 n=40+28)
RegexpMatchHard_1K-4     15.1MB/s ± 4%  15.1MB/s ± 3%  -0.11%  (p=0.015 n=40+40)
Revcomp-4                 137MB/s ± 3%   144MB/s ± 3%  +5.06%  (p=0.000 n=40+40)
Template-4               27.1MB/s ± 3%  27.1MB/s ± 4%    ~     (p=0.211 n=40+40)
[Geo mean]               78.9MB/s       79.7MB/s       +1.01%

Change-Id: I638fa4fef85833e8605919d693f9570cc3cf7334
Reviewed-on: https://go-review.googlesource.com/107275
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>