Use CMN/TST to simplify comparisons. This can reduce the
register pressure by removing single def/use registers for example:
ADDW R0, R1, R8 -> CMNW R1, R0 ; CMN is an alias of ADDS.
CBZW R8, label  -> BEQ  label  ; single def/use of R8 removed.

Little change in performance of go1 benchmark on Amberwing:
name                   old time/op    new time/op    delta
RegexpMatchEasy0_32       247ns ± 0%     246ns ± 0%  -0.40%  (p=0.008 n=5+5)
RegexpMatchEasy0_1K       581ns ± 0%     580ns ± 0%    ~     (p=0.079 n=4+5)
RegexpMatchEasy1_32       244ns ± 0%     243ns ± 0%  -0.41%  (p=0.008 n=5+5)
RegexpMatchEasy1_1K       804ns ± 0%     806ns ± 0%  +0.25%  (p=0.016 n=5+4)
RegexpMatchMedium_32      313ns ± 0%     311ns ± 0%  -0.64%  (p=0.008 n=5+5)
RegexpMatchMedium_1K     52.2µs ± 0%    51.9µs ± 0%  -0.51%  (p=0.008 n=5+5)
RegexpMatchHard_32       2.76µs ± 3%    2.74µs ± 0%    ~     (p=0.683 n=5+5)
RegexpMatchHard_1K       78.8µs ± 0%    78.9µs ± 0%  +0.04%  (p=0.008 n=5+5)
FmtFprintfEmpty          58.6ns ± 0%    57.7ns ± 0%  -1.54%  (p=0.008 n=5+5)
FmtFprintfString          118ns ± 0%     115ns ± 0%  -2.54%  (p=0.008 n=5+5)
FmtFprintfInt             119ns ± 0%     119ns ± 0%    ~     (all equal)
FmtFprintfIntInt          192ns ± 0%     192ns ± 0%    ~     (all equal)
FmtFprintfPrefixedInt     224ns ± 0%     205ns ± 0%  -8.48%  (p=0.008 n=5+5)
FmtFprintfFloat           336ns ± 0%     333ns ± 1%    ~     (p=0.683 n=5+5)
FmtManyArgs               779ns ± 1%     760ns ± 1%  -2.41%  (p=0.008 n=5+5)
Gzip                      437ms ± 0%     436ms ± 0%  -0.27%  (p=0.008 n=5+5)
HTTPClientServer         90.1µs ± 1%    91.1µs ± 0%  +1.19%  (p=0.008 n=5+5)
JSONEncode               20.1ms ± 0%    20.2ms ± 1%    ~     (p=0.690 n=5+5)
JSONDecode               94.5ms ± 1%    94.1ms ± 1%    ~     (p=0.095 n=5+5)
Mandelbrot200            5.37ms ± 0%    5.37ms ± 0%    ~     (p=0.421 n=5+5)
TimeParse                 450ns ± 0%     446ns ± 0%  -0.89%  (p=0.000 n=5+4)
TimeFormat                483ns ± 1%     473ns ± 0%  -2.19%  (p=0.008 n=5+5)
Template                 90.6ms ± 0%    89.7ms ± 0%  -0.93%  (p=0.008 n=5+5)
GoParse                  5.97ms ± 0%    6.01ms ± 0%  +0.65%  (p=0.008 n=5+5)
BinaryTree17              11.8s ± 0%     11.7s ± 0%  -0.28%  (p=0.016 n=5+5)
Revcomp                   669ms ± 0%     669ms ± 0%    ~     (p=0.222 n=5+5)
Fannkuch11                3.28s ± 0%     3.34s ± 0%  +1.72%  (p=0.016 n=4+5)
[Geo mean]               46.6µs         46.3µs       -0.74%

name                   old speed      new speed      delta
RegexpMatchEasy0_32     129MB/s ± 0%   130MB/s ± 0%  +0.32%  (p=0.016 n=5+4)
RegexpMatchEasy0_1K    1.76GB/s ± 0%  1.76GB/s ± 0%  +0.13%  (p=0.016 n=4+5)
RegexpMatchEasy1_32     131MB/s ± 0%   132MB/s ± 0%  +0.32%  (p=0.008 n=5+5)
RegexpMatchEasy1_1K    1.27GB/s ± 0%  1.27GB/s ± 0%  -0.24%  (p=0.016 n=5+4)
RegexpMatchMedium_32   3.19MB/s ± 0%  3.21MB/s ± 0%  +0.63%  (p=0.008 n=5+5)
RegexpMatchMedium_1K   19.6MB/s ± 0%  19.7MB/s ± 0%  +0.51%  (p=0.029 n=4+4)
RegexpMatchHard_32     11.6MB/s ± 2%  11.7MB/s ± 0%    ~     (p=1.000 n=5+5)
RegexpMatchHard_1K     13.0MB/s ± 0%  13.0MB/s ± 0%    ~     (p=0.079 n=4+5)
Gzip                   44.4MB/s ± 0%  44.5MB/s ± 0%  +0.27%  (p=0.008 n=5+5)
JSONEncode             96.4MB/s ± 0%  96.2MB/s ± 1%    ~     (p=0.579 n=5+5)
JSONDecode             20.5MB/s ± 1%  20.6MB/s ± 1%    ~     (p=0.111 n=5+5)
Template               21.4MB/s ± 0%  21.6MB/s ± 0%  +0.94%  (p=0.008 n=5+5)
GoParse                9.70MB/s ± 0%  9.63MB/s ± 0%  -0.68%  (p=0.016 n=4+5)
Revcomp                 380MB/s ± 0%   380MB/s ± 0%    ~     (p=0.222 n=5+5)
[Geo mean]             55.3MB/s       55.4MB/s       +0.23%

Change-Id: I2e5338138991d9bc984e67b51212aa5d1b0f2a6b
Reviewed-on: https://go-review.googlesource.com/97335Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>