CMPL/CMPW/CMPB can take a memory operand on 386, and this CL
implements that optimization.

1. The total size of pkg/linux_386 decreases about 45KB, excluding
cmd/compile.

2. The go1 benchmark shows a little improvement.
name                     old time/op    new time/op    delta
BinaryTree17-4              3.36s ± 2%     3.37s ± 3%    ~     (p=0.537 n=40+40)
Fannkuch11-4                3.59s ± 1%     3.53s ± 2%  -1.58%  (p=0.000 n=40+40)
FmtFprintfEmpty-4          46.0ns ± 3%    45.8ns ± 3%    ~     (p=0.249 n=40+40)
FmtFprintfString-4         80.0ns ± 4%    78.8ns ± 3%  -1.49%  (p=0.001 n=40+40)
FmtFprintfInt-4            89.7ns ± 2%    90.3ns ± 2%  +0.74%  (p=0.003 n=40+40)
FmtFprintfIntInt-4          144ns ± 3%     143ns ± 3%  -0.95%  (p=0.003 n=40+40)
FmtFprintfPrefixedInt-4     181ns ± 4%     180ns ± 2%    ~     (p=0.103 n=40+40)
FmtFprintfFloat-4           412ns ± 3%     408ns ± 4%  -0.97%  (p=0.018 n=40+40)
FmtManyArgs-4               607ns ± 4%     605ns ± 4%    ~     (p=0.148 n=40+40)
GobDecode-4                7.19ms ± 4%    7.24ms ± 5%    ~     (p=0.340 n=40+40)
GobEncode-4                7.04ms ± 9%    6.99ms ± 9%    ~     (p=0.289 n=40+40)
Gzip-4                      400ms ± 6%     398ms ± 5%    ~     (p=0.168 n=40+40)
Gunzip-4                   41.2ms ± 3%    41.7ms ± 3%  +1.40%  (p=0.001 n=40+40)
HTTPClientServer-4         62.5µs ± 1%    62.1µs ± 2%  -0.61%  (p=0.000 n=37+37)
JSONEncode-4               20.7ms ± 4%    20.4ms ± 3%  -1.60%  (p=0.000 n=40+40)
JSONDecode-4               69.4ms ± 4%    69.2ms ± 6%    ~     (p=0.177 n=40+40)
Mandelbrot200-4            5.22ms ± 6%    5.21ms ± 3%    ~     (p=0.531 n=40+40)
GoParse-4                  3.29ms ± 3%    3.28ms ± 3%    ~     (p=0.321 n=40+39)
RegexpMatchEasy0_32-4       104ns ± 4%     103ns ± 7%  -0.89%  (p=0.040 n=40+40)
RegexpMatchEasy0_1K-4       852ns ± 3%     853ns ± 2%    ~     (p=0.357 n=40+40)
RegexpMatchEasy1_32-4       113ns ± 8%     113ns ± 3%    ~     (p=0.906 n=40+40)
RegexpMatchEasy1_1K-4      1.03µs ± 4%    1.03µs ± 5%    ~     (p=0.326 n=40+40)
RegexpMatchMedium_32-4      136ns ± 3%     133ns ± 3%  -2.31%  (p=0.000 n=40+40)
RegexpMatchMedium_1K-4     44.0µs ± 3%    43.7µs ± 3%    ~     (p=0.053 n=40+40)
RegexpMatchHard_32-4       2.27µs ± 3%    2.26µs ± 4%    ~     (p=0.391 n=40+40)
RegexpMatchHard_1K-4       68.0µs ± 3%    68.9µs ± 3%  +1.28%  (p=0.000 n=40+40)
Revcomp-4                   1.86s ± 5%     1.86s ± 2%    ~     (p=0.950 n=40+40)
Template-4                 73.4ms ± 4%    69.9ms ± 7%  -4.78%  (p=0.000 n=40+40)
TimeParse-4                 449ns ± 4%     441ns ± 5%  -1.76%  (p=0.000 n=40+40)
TimeFormat-4                416ns ± 3%     417ns ± 4%    ~     (p=0.304 n=40+40)
[Geo mean]                 67.7µs         67.3µs       -0.55%

name                     old speed      new speed      delta
GobDecode-4               107MB/s ± 4%   106MB/s ± 5%    ~     (p=0.336 n=40+40)
GobEncode-4               109MB/s ± 5%   110MB/s ± 9%    ~     (p=0.142 n=38+40)
Gzip-4                   48.5MB/s ± 5%  48.8MB/s ± 5%    ~     (p=0.172 n=40+40)
Gunzip-4                  472MB/s ± 3%   465MB/s ± 3%  -1.39%  (p=0.001 n=40+40)
JSONEncode-4             93.6MB/s ± 4%  95.1MB/s ± 3%  +1.61%  (p=0.000 n=40+40)
JSONDecode-4             28.0MB/s ± 3%  28.1MB/s ± 6%    ~     (p=0.181 n=40+40)
GoParse-4                17.6MB/s ± 3%  17.7MB/s ± 3%    ~     (p=0.350 n=40+39)
RegexpMatchEasy0_32-4     308MB/s ± 4%   311MB/s ± 6%  +0.96%  (p=0.025 n=40+40)
RegexpMatchEasy0_1K-4    1.20GB/s ± 3%  1.20GB/s ± 2%    ~     (p=0.317 n=40+40)
RegexpMatchEasy1_32-4     282MB/s ± 7%   282MB/s ± 3%    ~     (p=0.516 n=40+40)
RegexpMatchEasy1_1K-4     994MB/s ± 4%   991MB/s ± 5%    ~     (p=0.319 n=40+40)
RegexpMatchMedium_32-4   7.31MB/s ± 3%  7.49MB/s ± 3%  +2.46%  (p=0.000 n=40+40)
RegexpMatchMedium_1K-4   23.3MB/s ± 3%  23.4MB/s ± 3%    ~     (p=0.052 n=40+40)
RegexpMatchHard_32-4     14.1MB/s ± 3%  14.1MB/s ± 4%    ~     (p=0.391 n=40+40)
RegexpMatchHard_1K-4     15.1MB/s ± 3%  14.9MB/s ± 3%  -1.27%  (p=0.000 n=40+40)
Revcomp-4                 137MB/s ± 5%   137MB/s ± 2%    ~     (p=0.942 n=40+40)
Template-4               26.5MB/s ± 4%  27.8MB/s ± 7%  +5.03%  (p=0.000 n=40+40)
[Geo mean]               78.6MB/s       79.0MB/s       +0.57%

Change-Id: Idcacc6881ef57cd7dc33aa87b711282842b72a53
Reviewed-on: https://go-review.googlesource.com/126618
Run-TryBot: Ben Shi <powerman1st@163.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>