"AND $0xffff0000, Rx" will be encoded to 12 bytes.
1. MOVWload from the constant pool to Rtmp
2. AND Rtmp, Rx
3. a 4-byte item in the constant pool

It can be simplified to 8 bytes on ARMv7, since ARMv7 has
"MOVW $imm-16, Rx".
1. MOVW $0xffff, Rtmp
2. BIC Rtmp, Rx

The above optimization also applies to BICconst, ADDconst and
SUBconst.

1. The total size of pkg/android_arm (excluding cmd/compile)
   decreases about 2KB.

2. The go1 benchmark shows no regression, exlcuding noise.
name                     old time/op    new time/op    delta
BinaryTree17-4              25.5s ± 1%     25.2s ± 1%  -0.85%  (p=0.000 n=30+30)
Fannkuch11-4                13.3s ± 0%     13.3s ± 0%  +0.16%  (p=0.000 n=24+25)
FmtFprintfEmpty-4           397ns ± 0%     394ns ± 0%  -0.64%  (p=0.000 n=30+30)
FmtFprintfString-4          679ns ± 0%     678ns ± 0%    ~     (p=0.093 n=30+29)
FmtFprintfInt-4             708ns ± 0%     707ns ± 0%  -0.19%  (p=0.000 n=27+28)
FmtFprintfIntInt-4         1.05µs ± 0%    1.05µs ± 0%  -0.07%  (p=0.001 n=18+30)
FmtFprintfPrefixedInt-4    1.16µs ± 0%    1.15µs ± 0%  -0.41%  (p=0.000 n=29+30)
FmtFprintfFloat-4          2.26µs ± 0%    2.23µs ± 1%  -1.40%  (p=0.000 n=30+30)
FmtManyArgs-4              3.96µs ± 0%    3.95µs ± 0%  -0.29%  (p=0.000 n=29+30)
GobDecode-4                52.9ms ± 2%    53.4ms ± 2%  +0.92%  (p=0.004 n=28+30)
GobEncode-4                49.7ms ± 2%    49.8ms ± 2%    ~     (p=0.890 n=30+26)
Gzip-4                      2.61s ± 0%     2.60s ± 0%  -0.36%  (p=0.000 n=29+29)
Gunzip-4                    312ms ± 0%     311ms ± 0%  -0.13%  (p=0.000 n=30+28)
HTTPClientServer-4         1.02ms ± 8%    1.00ms ± 7%    ~     (p=0.224 n=29+26)
JSONEncode-4                125ms ± 1%     124ms ± 3%  -1.05%  (p=0.000 n=25+30)
JSONDecode-4                432ms ± 1%     436ms ± 2%    ~     (p=0.277 n=26+30)
Mandelbrot200-4            18.4ms ± 0%    18.4ms ± 0%  +0.02%  (p=0.001 n=28+25)
GoParse-4                  22.4ms ± 1%    22.3ms ± 1%  -0.41%  (p=0.000 n=28+28)
RegexpMatchEasy0_32-4       697ns ± 0%     706ns ± 0%  +1.23%  (p=0.000 n=19+30)
RegexpMatchEasy0_1K-4      4.27µs ± 0%    4.26µs ± 0%  -0.06%  (p=0.000 n=30+30)
RegexpMatchEasy1_32-4       741ns ± 0%     735ns ± 0%  -0.86%  (p=0.000 n=26+30)
RegexpMatchEasy1_1K-4      5.49µs ± 0%    5.49µs ± 0%  -0.03%  (p=0.023 n=25+30)
RegexpMatchMedium_32-4     1.05µs ± 2%    1.04µs ± 2%    ~     (p=0.893 n=30+30)
RegexpMatchMedium_1K-4      261µs ± 0%     261µs ± 0%  -0.11%  (p=0.000 n=29+30)
RegexpMatchHard_32-4       14.9µs ± 0%    14.9µs ± 0%  -0.36%  (p=0.000 n=23+29)
RegexpMatchHard_1K-4        446µs ± 0%     445µs ± 0%  -0.17%  (p=0.000 n=30+29)
Revcomp-4                  41.6ms ± 1%    41.7ms ± 1%  +0.27%  (p=0.040 n=28+30)
Template-4                  531ms ± 0%     532ms ± 1%    ~     (p=0.059 n=30+30)
TimeParse-4                3.40µs ± 0%    3.33µs ± 0%  -2.02%  (p=0.000 n=30+30)
TimeFormat-4               6.14µs ± 0%    6.11µs ± 0%  -0.45%  (p=0.000 n=27+29)
[Geo mean]                  384µs          383µs       -0.27%

name                     old speed      new speed      delta
GobDecode-4              14.5MB/s ± 2%  14.4MB/s ± 2%  -0.90%  (p=0.005 n=28+30)
GobEncode-4              15.4MB/s ± 2%  15.4MB/s ± 2%    ~     (p=0.741 n=30+25)
Gzip-4                   7.44MB/s ± 0%  7.47MB/s ± 1%  +0.37%  (p=0.000 n=25+30)
Gunzip-4                 62.3MB/s ± 0%  62.4MB/s ± 0%  +0.13%  (p=0.000 n=30+28)
JSONEncode-4             15.5MB/s ± 1%  15.6MB/s ± 3%  +1.07%  (p=0.000 n=25+30)
JSONDecode-4             4.48MB/s ± 0%  4.46MB/s ± 2%    ~     (p=0.655 n=23+30)
GoParse-4                2.58MB/s ± 1%  2.59MB/s ± 1%  +0.42%  (p=0.000 n=28+29)
RegexpMatchEasy0_32-4    45.9MB/s ± 0%  45.3MB/s ± 0%  -1.23%  (p=0.000 n=28+30)
RegexpMatchEasy0_1K-4     240MB/s ± 0%   240MB/s ± 0%  +0.07%  (p=0.000 n=30+30)
RegexpMatchEasy1_32-4    43.2MB/s ± 0%  43.5MB/s ± 0%  +0.85%  (p=0.000 n=30+28)
RegexpMatchEasy1_1K-4     186MB/s ± 0%   186MB/s ± 0%  +0.03%  (p=0.026 n=25+30)
RegexpMatchMedium_32-4    955kB/s ± 2%   960kB/s ± 2%    ~     (p=0.084 n=30+30)
RegexpMatchMedium_1K-4   3.92MB/s ± 0%  3.93MB/s ± 0%  +0.14%  (p=0.000 n=29+30)
RegexpMatchHard_32-4     2.14MB/s ± 0%  2.15MB/s ± 0%  +0.31%  (p=0.000 n=30+26)
RegexpMatchHard_1K-4     2.30MB/s ± 0%  2.30MB/s ± 0%    ~     (all equal)
Revcomp-4                61.1MB/s ± 1%  60.9MB/s ± 1%  -0.27%  (p=0.039 n=28+30)
Template-4               3.66MB/s ± 0%  3.65MB/s ± 1%  -0.14%  (p=0.045 n=30+30)
[Geo mean]               12.8MB/s       12.8MB/s       +0.04%

Change-Id: I02370e2584b4c041fddd324c97628fd6f0c12183
Reviewed-on: https://go-review.googlesource.com/123179
Run-TryBot: Ben Shi <powerman1st@163.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>