Currently "ADD $0x123456, Rs, Rd" will load pre-stored 0x123456
from the constant pool and use it for the addition. Total 12 bytes
are cost. And so does SUB.

This CL breaks it to "ADD 0x123000, Rs, Rd" + "ADD 0x000456, Rd, Rd".
Both "0x123000" and "0x000456" can be directly encoded into the
instruction binary code. So 4 bytes are saved.

1. The total size of pkg/android_arm64 decreases about 0.3KB.

2. The go1 benchmark show little regression (excluding noise).

name                     old time/op    new time/op    delta
BinaryTree17-4              15.9s ± 0%     15.9s ± 1%  +0.10%  (p=0.044 n=29+29)
Fannkuch11-4                8.72s ± 0%     8.75s ± 0%  +0.34%  (p=0.000 n=30+24)
FmtFprintfEmpty-4           173ns ± 0%     173ns ± 0%    ~     (all equal)
FmtFprintfString-4          368ns ± 0%     368ns ± 0%    ~     (p=0.593 n=30+30)
FmtFprintfInt-4             417ns ± 0%     417ns ± 0%    ~     (all equal)
FmtFprintfIntInt-4          673ns ± 0%     661ns ± 1%  -1.70%  (p=0.000 n=30+30)
FmtFprintfPrefixedInt-4     805ns ± 0%     805ns ± 0%  +0.10%  (p=0.011 n=30+30)
FmtFprintfFloat-4          1.09µs ± 0%    1.09µs ± 0%    ~     (p=0.125 n=30+29)
FmtManyArgs-4              2.68µs ± 0%    2.68µs ± 0%  +0.07%  (p=0.004 n=30+30)
GobDecode-4                32.9ms ± 0%    33.2ms ± 1%  +1.07%  (p=0.000 n=29+29)
GobEncode-4                29.5ms ± 0%    29.6ms ± 0%  +0.26%  (p=0.000 n=28+28)
Gzip-4                      1.38s ± 1%     1.35s ± 3%  -1.94%  (p=0.000 n=28+30)
Gunzip-4                    139ms ± 0%     139ms ± 0%  +0.10%  (p=0.000 n=28+29)
HTTPClientServer-4          745µs ± 5%     742µs ± 3%    ~     (p=0.405 n=28+29)
JSONEncode-4               49.5ms ± 1%    49.9ms ± 0%  +0.89%  (p=0.000 n=30+30)
JSONDecode-4                264ms ± 1%     264ms ± 0%  +0.25%  (p=0.001 n=30+30)
Mandelbrot200-4            16.6ms ± 0%    16.6ms ± 0%    ~     (p=0.507 n=29+29)
GoParse-4                  15.9ms ± 0%    16.0ms ± 1%  +0.91%  (p=0.002 n=23+30)
RegexpMatchEasy0_32-4       379ns ± 0%     379ns ± 0%    ~     (all equal)
RegexpMatchEasy0_1K-4      1.31µs ± 0%    1.31µs ± 0%  +0.09%  (p=0.008 n=27+30)
RegexpMatchEasy1_32-4       357ns ± 0%     358ns ± 0%  +0.28%  (p=0.000 n=28+29)
RegexpMatchEasy1_1K-4      2.04µs ± 0%    2.04µs ± 0%    ~     (p=0.850 n=30+30)
RegexpMatchMedium_32-4      587ns ± 0%     589ns ± 0%  +0.33%  (p=0.000 n=30+30)
RegexpMatchMedium_1K-4      162µs ± 0%     163µs ± 0%    ~     (p=0.351 n=30+29)
RegexpMatchHard_32-4       9.54µs ± 0%    9.60µs ± 0%  +0.59%  (p=0.000 n=28+30)
RegexpMatchHard_1K-4        287µs ± 0%     287µs ± 0%  +0.11%  (p=0.000 n=26+29)
Revcomp-4                   2.50s ± 0%     2.50s ± 0%  -0.13%  (p=0.012 n=28+27)
Template-4                  312ms ± 1%     312ms ± 1%  +0.20%  (p=0.015 n=27+30)
TimeParse-4                1.68µs ± 0%    1.68µs ± 0%  -0.35%  (p=0.000 n=30+30)
TimeFormat-4               1.66µs ± 0%    1.64µs ± 0%  -1.20%  (p=0.000 n=25+29)
[Geo mean]                  246µs          246µs       -0.00%

name                     old speed      new speed      delta
GobDecode-4              23.3MB/s ± 0%  23.1MB/s ± 1%  -1.05%  (p=0.000 n=29+29)
GobEncode-4              26.0MB/s ± 0%  25.9MB/s ± 0%  -0.25%  (p=0.000 n=29+28)
Gzip-4                   14.1MB/s ± 1%  14.4MB/s ± 3%  +1.94%  (p=0.000 n=27+30)
Gunzip-4                  139MB/s ± 0%   139MB/s ± 0%  -0.10%  (p=0.000 n=28+29)
JSONEncode-4             39.2MB/s ± 1%  38.9MB/s ± 0%  -0.88%  (p=0.000 n=30+30)
JSONDecode-4             7.37MB/s ± 0%  7.35MB/s ± 0%  -0.26%  (p=0.001 n=30+30)
GoParse-4                3.65MB/s ± 0%  3.62MB/s ± 1%  -0.86%  (p=0.001 n=23+30)
RegexpMatchEasy0_32-4    84.3MB/s ± 0%  84.3MB/s ± 0%    ~     (p=0.126 n=27+26)
RegexpMatchEasy0_1K-4     784MB/s ± 0%   783MB/s ± 0%  -0.10%  (p=0.003 n=27+30)
RegexpMatchEasy1_32-4    89.5MB/s ± 0%  89.3MB/s ± 0%  -0.20%  (p=0.000 n=27+29)
RegexpMatchEasy1_1K-4     502MB/s ± 0%   502MB/s ± 0%    ~     (p=0.858 n=30+28)
RegexpMatchMedium_32-4   1.70MB/s ± 0%  1.70MB/s ± 0%  -0.25%  (p=0.000 n=30+30)
RegexpMatchMedium_1K-4   6.30MB/s ± 0%  6.30MB/s ± 0%    ~     (all equal)
RegexpMatchHard_32-4     3.35MB/s ± 0%  3.33MB/s ± 0%  -0.47%  (p=0.000 n=30+30)
RegexpMatchHard_1K-4     3.57MB/s ± 0%  3.56MB/s ± 0%  -0.20%  (p=0.000 n=27+30)
Revcomp-4                 102MB/s ± 0%   102MB/s ± 0%  +0.14%  (p=0.008 n=28+28)
Template-4               6.23MB/s ± 0%  6.21MB/s ± 1%  -0.21%  (p=0.009 n=21+30)
[Geo mean]               24.1MB/s       24.0MB/s       -0.16%

Change-Id: Ifcef3edb667540e2d86e586c23afcfbc2cf1340b
Reviewed-on: https://go-review.googlesource.com/c/134536
Run-TryBot: Ben Shi <powerman1st@163.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>