This CL adds more combinations of two consequtive MOVBload/MOVBstore
to a unique MOVWload/MOVWstore.

1. The size of the go executable decreases about 4KB, and the total
size of pkg/linux_386 (excluding cmd/compile) decreases about 1.5KB.

2. There is no regression in the go1 benchmark result, excluding noise.
name                     old time/op    new time/op    delta
BinaryTree17-4              3.28s ± 2%     3.29s ± 2%    ~     (p=0.151 n=40+40)
Fannkuch11-4                3.52s ± 1%     3.51s ± 1%  -0.28%  (p=0.002 n=40+40)
FmtFprintfEmpty-4          45.4ns ± 4%    45.0ns ± 4%  -0.89%  (p=0.019 n=40+40)
FmtFprintfString-4         81.9ns ± 7%    81.3ns ± 1%    ~     (p=0.660 n=40+25)
FmtFprintfInt-4            91.9ns ± 9%    91.4ns ± 9%    ~     (p=0.249 n=40+40)
FmtFprintfIntInt-4          143ns ± 4%     143ns ± 4%    ~     (p=0.760 n=40+40)
FmtFprintfPrefixedInt-4     184ns ± 3%     183ns ± 4%    ~     (p=0.485 n=40+40)
FmtFprintfFloat-4           408ns ± 3%     409ns ± 3%    ~     (p=0.961 n=40+40)
FmtManyArgs-4               597ns ± 4%     602ns ± 3%    ~     (p=0.413 n=40+40)
GobDecode-4                7.13ms ± 6%    7.14ms ± 6%    ~     (p=0.859 n=40+40)
GobEncode-4                6.86ms ± 9%    6.94ms ± 7%    ~     (p=0.162 n=40+40)
Gzip-4                      395ms ± 4%     396ms ± 3%    ~     (p=0.099 n=40+40)
Gunzip-4                   40.9ms ± 4%    41.1ms ± 3%    ~     (p=0.064 n=40+40)
HTTPClientServer-4         63.6µs ± 2%    63.6µs ± 3%    ~     (p=0.832 n=36+39)
JSONEncode-4               16.1ms ± 3%    15.8ms ± 3%  -1.60%  (p=0.001 n=40+40)
JSONDecode-4               61.0ms ± 3%    61.5ms ± 4%    ~     (p=0.065 n=40+40)
Mandelbrot200-4            5.16ms ± 3%    5.18ms ± 3%    ~     (p=0.056 n=40+40)
GoParse-4                  3.25ms ± 2%    3.23ms ± 3%    ~     (p=0.727 n=40+40)
RegexpMatchEasy0_32-4      90.2ns ± 3%    89.3ns ± 6%  -0.98%  (p=0.002 n=40+40)
RegexpMatchEasy0_1K-4       812ns ± 3%     815ns ± 3%    ~     (p=0.309 n=40+40)
RegexpMatchEasy1_32-4       103ns ± 6%     103ns ± 5%    ~     (p=0.680 n=40+40)
RegexpMatchEasy1_1K-4      1.01µs ± 4%    1.02µs ± 3%    ~     (p=0.326 n=40+33)
RegexpMatchMedium_32-4      120ns ± 4%     120ns ± 5%    ~     (p=0.834 n=40+40)
RegexpMatchMedium_1K-4     40.1µs ± 3%    39.5µs ± 4%  -1.35%  (p=0.000 n=40+40)
RegexpMatchHard_32-4       2.27µs ± 6%    2.23µs ± 4%  -1.67%  (p=0.011 n=40+40)
RegexpMatchHard_1K-4       67.2µs ± 3%    67.2µs ± 3%    ~     (p=0.149 n=40+40)
Revcomp-4                   1.84s ± 2%     1.86s ± 3%  +0.70%  (p=0.020 n=40+40)
Template-4                 69.0ms ± 4%    69.8ms ± 3%  +1.20%  (p=0.003 n=40+40)
TimeParse-4                 438ns ± 3%     439ns ± 4%    ~     (p=0.650 n=40+40)
TimeFormat-4                412ns ± 3%     412ns ± 3%    ~     (p=0.888 n=40+40)
[Geo mean]                 65.2µs         65.2µs       -0.04%

name                     old speed      new speed      delta
GobDecode-4               108MB/s ± 6%   108MB/s ± 6%    ~     (p=0.855 n=40+40)
GobEncode-4               112MB/s ± 9%   111MB/s ± 8%    ~     (p=0.159 n=40+40)
Gzip-4                   49.2MB/s ± 4%  49.1MB/s ± 3%    ~     (p=0.102 n=40+40)
Gunzip-4                  474MB/s ± 3%   472MB/s ± 3%    ~     (p=0.063 n=40+40)
JSONEncode-4              121MB/s ± 3%   123MB/s ± 3%  +1.62%  (p=0.001 n=40+40)
JSONDecode-4             31.9MB/s ± 3%  31.6MB/s ± 4%    ~     (p=0.070 n=40+40)
GoParse-4                17.9MB/s ± 2%  17.9MB/s ± 3%    ~     (p=0.696 n=40+40)
RegexpMatchEasy0_32-4     355MB/s ± 3%   358MB/s ± 5%  +0.99%  (p=0.002 n=40+40)
RegexpMatchEasy0_1K-4    1.26GB/s ± 3%  1.26GB/s ± 3%    ~     (p=0.381 n=40+40)
RegexpMatchEasy1_32-4     310MB/s ± 5%   310MB/s ± 4%    ~     (p=0.655 n=40+40)
RegexpMatchEasy1_1K-4    1.01GB/s ± 4%  1.01GB/s ± 3%    ~     (p=0.351 n=40+33)
RegexpMatchMedium_32-4   8.32MB/s ± 4%  8.34MB/s ± 5%    ~     (p=0.696 n=40+40)
RegexpMatchMedium_1K-4   25.6MB/s ± 3%  25.9MB/s ± 4%  +1.36%  (p=0.000 n=40+40)
RegexpMatchHard_32-4     14.1MB/s ± 6%  14.3MB/s ± 4%  +1.64%  (p=0.011 n=40+40)
RegexpMatchHard_1K-4     15.2MB/s ± 3%  15.2MB/s ± 3%    ~     (p=0.147 n=40+40)
Revcomp-4                 138MB/s ± 2%   137MB/s ± 3%  -0.70%  (p=0.021 n=40+40)
Template-4               28.1MB/s ± 4%  27.8MB/s ± 3%  -1.19%  (p=0.003 n=40+40)
[Geo mean]               83.7MB/s       83.7MB/s       +0.03%

Change-Id: I2a2b3a942b5c45467491515d201179fd192e65c9
Reviewed-on: https://go-review.googlesource.com/c/141650
Run-TryBot: Ben Shi <powerman1st@163.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>