Current assembler saves constants in Offset which type is int64,
causing 32-bit constants have a incorrect class. This CL reclassifies
constants when opcodes are 32-bit variant, like MOVW, ANDW and
ADDW, etc. Besides, this CL encodes some constants of ADDCON class
as MOVs instructions.

This CL changes the assembler behavior as follows.

1. go assembler ADDW $MOVCON, Rn, Rd
   previous version: MOVD $MOVCON, Rtmp; ADDW Rtmp, Rn, Rd
   current version: MOVW $MOVCON, Rtmp; ADDW Rtmp, Rn, Rd

2. go assembly MOVW $0xaaaaffff, R1
   previous version: treats $0xaaaaffff as VCON, encodes it as MOVW 0x994, R1 (loads it from pool).
   current version: treats $0xaaaaffff as MOVCON, and encodes it into MOVW instructions.

3. go assembly MOVD $0x210000, R1
   previous version: treats $0x210000 as ADDCON, loads it from pool
   current version: treats $0x210000 as MOVCON, and encodes it into MOVD instructions.

Add the test cases.

1. Binary size before/after.
binary                          size change
pkg/linux_arm64                 -1.534KB
pkg/tool/linux_arm64            -0.718KB
go                              -0.32KB
gofmt                           no change

2. go1 benchmark result.
name                     old time/op    new time/op    delta
BinaryTree17-8              6.26s ± 1%     6.28s ± 1%     ~     (p=0.105 n=10+10)
Fannkuch11-8                5.40s ± 0%     5.39s ± 0%   -0.29%  (p=0.028 n=9+10)
FmtFprintfEmpty-8          94.5ns ± 0%    95.0ns ± 0%   +0.51%  (p=0.000 n=10+9)
FmtFprintfString-8          163ns ± 1%     159ns ± 1%   -2.06%  (p=0.000 n=10+9)
FmtFprintfInt-8             200ns ± 1%     196ns ± 1%   -1.99%  (p=0.000 n=9+10)
FmtFprintfIntInt-8          292ns ± 3%     284ns ± 1%   -2.87%  (p=0.001 n=10+9)
FmtFprintfPrefixedInt-8     422ns ± 1%     420ns ± 1%   -0.59%  (p=0.015 n=10+10)
FmtFprintfFloat-8           458ns ± 0%     463ns ± 1%   +1.19%  (p=0.000 n=9+10)
FmtManyArgs-8              1.37µs ± 1%    1.35µs ± 1%   -1.85%  (p=0.000 n=10+10)
GobDecode-8                15.5ms ± 1%    15.3ms ± 1%   -1.82%  (p=0.000 n=10+10)
GobEncode-8                11.7ms ± 5%    11.7ms ± 2%     ~     (p=0.549 n=10+9)
Gzip-8                      622ms ± 0%     624ms ± 0%   +0.23%  (p=0.000 n=10+9)
Gunzip-8                   73.6ms ± 0%    73.8ms ± 1%     ~     (p=0.077 n=9+9)
HTTPClientServer-8          115µs ± 1%     115µs ± 1%     ~     (p=0.796 n=10+10)
JSONEncode-8               31.1ms ± 2%    28.7ms ± 1%   -7.98%  (p=0.000 n=10+9)
JSONDecode-8                145ms ± 0%     145ms ± 1%     ~     (p=0.447 n=9+10)
Mandelbrot200-8            9.67ms ± 0%    9.60ms ± 0%   -0.76%  (p=0.000 n=9+9)
GoParse-8                  7.56ms ± 1%    7.58ms ± 0%   +0.21%  (p=0.035 n=10+9)
RegexpMatchEasy0_32-8       208ns ±10%     222ns ± 0%     ~     (p=0.531 n=10+6)
RegexpMatchEasy0_1K-8       699ns ± 4%     694ns ± 4%     ~     (p=0.868 n=10+10)
RegexpMatchEasy1_32-8       186ns ± 8%     190ns ±12%     ~     (p=0.955 n=10+10)
RegexpMatchEasy1_1K-8      1.13µs ± 1%    1.05µs ± 2%   -6.64%  (p=0.000 n=10+10)
RegexpMatchMedium_32-8      316ns ± 7%     288ns ± 1%   -8.68%  (p=0.000 n=10+7)
RegexpMatchMedium_1K-8     90.2µs ± 0%    85.5µs ± 2%   -5.19%  (p=0.000 n=10+10)
RegexpMatchHard_32-8       5.53µs ± 0%    3.90µs ± 0%  -29.52%  (p=0.000 n=10+10)
RegexpMatchHard_1K-8        119µs ± 0%     124µs ± 0%   +4.29%  (p=0.000 n=9+10)
Revcomp-8                   1.07s ± 0%     1.07s ± 0%     ~     (p=0.094 n=9+9)
Template-8                  162ms ± 1%     160ms ± 2%     ~     (p=0.089 n=10+10)
TimeParse-8                 756ns ± 2%     763ns ± 1%     ~     (p=0.158 n=10+10)
TimeFormat-8                758ns ± 1%     746ns ± 1%   -1.52%  (p=0.000 n=10+10)

name                     old speed      new speed      delta
GobDecode-8              49.4MB/s ± 1%  50.3MB/s ± 1%   +1.84%  (p=0.000 n=10+10)
GobEncode-8              65.6MB/s ± 5%  65.4MB/s ± 2%     ~     (p=0.549 n=10+9)
Gzip-8                   31.2MB/s ± 0%  31.1MB/s ± 0%   -0.24%  (p=0.000 n=9+9)
Gunzip-8                  264MB/s ± 0%   263MB/s ± 1%     ~     (p=0.073 n=9+9)
JSONEncode-8             62.3MB/s ± 2%  67.7MB/s ± 1%   +8.67%  (p=0.000 n=10+9)
JSONDecode-8             13.4MB/s ± 0%  13.4MB/s ± 1%     ~     (p=0.508 n=9+10)
GoParse-8                7.66MB/s ± 1%  7.64MB/s ± 0%   -0.23%  (p=0.049 n=10+9)
RegexpMatchEasy0_32-8     154MB/s ± 9%   143MB/s ± 3%     ~     (p=0.303 n=10+7)
RegexpMatchEasy0_1K-8    1.46GB/s ± 4%  1.47GB/s ± 4%     ~     (p=0.912 n=10+10)
RegexpMatchEasy1_32-8     172MB/s ± 9%   170MB/s ±12%     ~     (p=0.971 n=10+10)
RegexpMatchEasy1_1K-8     908MB/s ± 1%   972MB/s ± 2%   +7.12%  (p=0.000 n=10+10)
RegexpMatchMedium_32-8   3.17MB/s ± 7%  3.46MB/s ± 1%   +9.14%  (p=0.000 n=10+7)
RegexpMatchMedium_1K-8   11.3MB/s ± 0%  12.0MB/s ± 2%   +5.51%  (p=0.000 n=10+10)
RegexpMatchHard_32-8     5.78MB/s ± 0%  8.21MB/s ± 0%  +41.93%  (p=0.000 n=9+10)
RegexpMatchHard_1K-8     8.62MB/s ± 0%  8.27MB/s ± 0%   -4.11%  (p=0.000 n=9+10)
Revcomp-8                 237MB/s ± 0%   237MB/s ± 0%     ~     (p=0.081 n=9+9)
Template-8               12.0MB/s ± 1%  12.1MB/s ± 2%     ~     (p=0.072 n=10+10)

Change-Id: I080801f520366b42d5f9699954bd33106976a81b
Reviewed-on: https://go-review.googlesource.com/c/120661
Run-TryBot: Ben Shi <powerman1st@163.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>