This CL optimize amd64's code:
"ADDQ $-1, MEM_OP" -> "DECQ MEM_OP"
"ADDL $-1, MEM_OP" -> "DECL MEM_OP"

1. The total size of pkg/linux_amd64 (excluding cmd/compile)
decreases about 0.1KB.

2. The go1 benchmark shows little regression, excluding noise.
name                     old time/op    new time/op    delta
BinaryTree17-4              2.60s ± 5%     2.64s ± 3%  +1.53%  (p=0.000 n=38+39)
Fannkuch11-4                2.37s ± 2%     2.38s ± 2%    ~     (p=0.950 n=40+40)
FmtFprintfEmpty-4          40.4ns ± 5%    40.5ns ± 5%    ~     (p=0.711 n=40+40)
FmtFprintfString-4         72.4ns ± 5%    72.3ns ± 3%    ~     (p=0.485 n=40+40)
FmtFprintfInt-4            79.7ns ± 3%    80.1ns ± 3%    ~     (p=0.124 n=40+40)
FmtFprintfIntInt-4          126ns ± 3%     127ns ± 3%  +0.71%  (p=0.027 n=40+40)
FmtFprintfPrefixedInt-4     153ns ± 4%     153ns ± 2%    ~     (p=0.604 n=40+40)
FmtFprintfFloat-4           206ns ± 5%     210ns ± 5%  +1.79%  (p=0.002 n=40+40)
FmtManyArgs-4               498ns ± 3%     496ns ± 3%    ~     (p=0.099 n=40+40)
GobDecode-4                6.48ms ± 6%    6.47ms ± 7%    ~     (p=0.686 n=39+40)
GobEncode-4                5.95ms ± 7%    5.96ms ± 6%    ~     (p=0.670 n=40+34)
Gzip-4                      224ms ± 6%     223ms ± 5%    ~     (p=0.143 n=40+40)
Gunzip-4                   36.5ms ± 4%    36.5ms ± 4%    ~     (p=0.556 n=40+40)
HTTPClientServer-4         60.7µs ± 2%    59.9µs ± 3%  -1.20%  (p=0.000 n=39+39)
JSONEncode-4               9.03ms ± 4%    9.04ms ± 4%    ~     (p=0.589 n=40+40)
JSONDecode-4               49.4ms ± 4%    49.2ms ± 4%    ~     (p=0.276 n=40+40)
Mandelbrot200-4            3.80ms ± 4%    3.79ms ± 4%    ~     (p=0.837 n=40+40)
GoParse-4                  3.15ms ± 5%    3.13ms ± 5%    ~     (p=0.240 n=40+40)
RegexpMatchEasy0_32-4      72.9ns ± 3%    72.0ns ± 8%  -1.25%  (p=0.003 n=40+40)
RegexpMatchEasy0_1K-4       229ns ± 5%     230ns ± 4%    ~     (p=0.318 n=40+40)
RegexpMatchEasy1_32-4      66.9ns ± 3%    67.3ns ± 7%    ~     (p=0.817 n=40+40)
RegexpMatchEasy1_1K-4       371ns ± 5%     370ns ± 4%    ~     (p=0.275 n=40+40)
RegexpMatchMedium_32-4      106ns ± 4%     104ns ± 7%  -2.28%  (p=0.000 n=40+40)
RegexpMatchMedium_1K-4     32.0µs ± 2%    31.4µs ± 3%  -2.08%  (p=0.000 n=40+40)
RegexpMatchHard_32-4       1.54µs ± 7%    1.52µs ± 3%  -1.80%  (p=0.007 n=39+40)
RegexpMatchHard_1K-4       45.8µs ± 4%    45.5µs ± 3%    ~     (p=0.707 n=40+40)
Revcomp-4                   401ms ± 5%     401ms ± 6%    ~     (p=0.935 n=40+40)
Template-4                 62.4ms ± 4%    61.2ms ± 3%  -1.85%  (p=0.000 n=40+40)
TimeParse-4                 315ns ± 2%     318ns ± 3%  +1.10%  (p=0.002 n=40+40)
TimeFormat-4                297ns ± 3%     298ns ± 3%    ~     (p=0.238 n=40+40)
[Geo mean]                 45.8µs         45.7µs       -0.22%

name                     old speed      new speed      delta
GobDecode-4               119MB/s ± 6%   119MB/s ± 7%    ~     (p=0.684 n=39+40)
GobEncode-4               129MB/s ± 7%   128MB/s ± 6%    ~     (p=0.413 n=40+34)
Gzip-4                   86.6MB/s ± 6%  87.0MB/s ± 6%    ~     (p=0.145 n=40+40)
Gunzip-4                  532MB/s ± 4%   532MB/s ± 4%    ~     (p=0.556 n=40+40)
JSONEncode-4              215MB/s ± 4%   215MB/s ± 4%    ~     (p=0.583 n=40+40)
JSONDecode-4             39.3MB/s ± 4%  39.5MB/s ± 4%    ~     (p=0.277 n=40+40)
GoParse-4                18.4MB/s ± 5%  18.5MB/s ± 5%    ~     (p=0.229 n=40+40)
RegexpMatchEasy0_32-4     439MB/s ± 3%   445MB/s ± 8%  +1.28%  (p=0.003 n=40+40)
RegexpMatchEasy0_1K-4    4.46GB/s ± 4%  4.45GB/s ± 4%    ~     (p=0.343 n=40+40)
RegexpMatchEasy1_32-4     479MB/s ± 3%   476MB/s ± 7%    ~     (p=0.855 n=40+40)
RegexpMatchEasy1_1K-4    2.76GB/s ± 5%  2.77GB/s ± 4%    ~     (p=0.250 n=40+40)
RegexpMatchMedium_32-4   9.36MB/s ± 4%  9.58MB/s ± 6%  +2.31%  (p=0.001 n=40+40)
RegexpMatchMedium_1K-4   32.0MB/s ± 2%  32.7MB/s ± 3%  +2.12%  (p=0.000 n=40+40)
RegexpMatchHard_32-4     20.7MB/s ± 7%  21.1MB/s ± 3%  +1.95%  (p=0.005 n=40+40)
RegexpMatchHard_1K-4     22.4MB/s ± 4%  22.5MB/s ± 3%    ~     (p=0.689 n=40+40)
Revcomp-4                 634MB/s ± 5%   634MB/s ± 6%    ~     (p=0.935 n=40+40)
Template-4               31.1MB/s ± 3%  31.7MB/s ± 3%  +1.88%  (p=0.000 n=40+40)
[Geo mean]                129MB/s        130MB/s       +0.62%

Change-Id: I9d61ee810d900920c572cbe89e2f1626bfed12b7
Reviewed-on: https://go-review.googlesource.com/c/145209
Run-TryBot: Ben Shi <powerman1st@163.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>