When lowering some ops on amd64 we generate additional NEGQ.
This may result in code like this:

NEGQ R12
NEGQ R12

Optimize it away. Gain is not significant, about ~0.5% gain in geomean
in compress/flate and 200 bytes codesize reduction in go tool.

Full results below:

name                             old time/op    new time/op    delta
Encode/Digits/Huffman/1e4-6        65.8µs ± 0%    65.7µs ± 0%  -0.21%  (p=0.010 n=10+9)
Encode/Digits/Huffman/1e5-6         633µs ± 0%     632µs ± 0%    ~     (p=0.370 n=8+9)
Encode/Digits/Huffman/1e6-6        6.30ms ± 1%    6.29ms ± 1%    ~     (p=0.796 n=10+10)
Encode/Digits/Speed/1e4-6           281µs ± 0%     280µs ± 1%  -0.34%  (p=0.043 n=8+10)
Encode/Digits/Speed/1e5-6          2.66ms ± 0%    2.66ms ± 0%  -0.09%  (p=0.043 n=10+10)
Encode/Digits/Speed/1e6-6          26.3ms ± 0%    26.3ms ± 0%    ~     (p=0.190 n=10+10)
Encode/Digits/Default/1e4-6         554µs ± 0%     557µs ± 0%  +0.46%  (p=0.001 n=9+10)
Encode/Digits/Default/1e5-6        8.63ms ± 1%    8.62ms ± 1%    ~     (p=0.912 n=10+10)
Encode/Digits/Default/1e6-6        92.7ms ± 1%    92.2ms ± 1%    ~     (p=0.052 n=10+10)
Encode/Digits/Compression/1e4-6     558µs ± 1%     557µs ± 1%    ~     (p=0.481 n=10+10)
Encode/Digits/Compression/1e5-6    8.58ms ± 0%    8.61ms ± 1%    ~     (p=0.315 n=8+10)
Encode/Digits/Compression/1e6-6    92.3ms ± 1%    92.4ms ± 1%    ~     (p=0.971 n=10+10)
Encode/Twain/Huffman/1e4-6         89.5µs ± 0%    89.0µs ± 1%  -0.48%  (p=0.001 n=9+9)
Encode/Twain/Huffman/1e5-6          727µs ± 1%     728µs ± 0%    ~     (p=0.604 n=10+9)
Encode/Twain/Huffman/1e6-6         7.21ms ± 0%    7.19ms ± 1%    ~     (p=0.696 n=8+10)
Encode/Twain/Speed/1e4-6            320µs ± 1%     321µs ± 1%    ~     (p=0.353 n=10+10)
Encode/Twain/Speed/1e5-6           2.63ms ± 0%    2.62ms ± 1%  -0.33%  (p=0.016 n=8+10)
Encode/Twain/Speed/1e6-6           25.8ms ± 0%    25.8ms ± 0%    ~     (p=0.360 n=10+8)
Encode/Twain/Default/1e4-6          677µs ± 1%     671µs ± 1%  -0.88%  (p=0.000 n=10+10)
Encode/Twain/Default/1e5-6         10.5ms ± 1%    10.3ms ± 0%  -2.06%  (p=0.000 n=10+10)
Encode/Twain/Default/1e6-6          113ms ± 1%     111ms ± 1%  -1.96%  (p=0.000 n=10+9)
Encode/Twain/Compression/1e4-6      688µs ± 0%     679µs ± 1%  -1.30%  (p=0.000 n=7+10)
Encode/Twain/Compression/1e5-6     11.6ms ± 1%    11.3ms ± 1%  -2.10%  (p=0.000 n=10+10)
Encode/Twain/Compression/1e6-6      126ms ± 1%     124ms ± 0%  -1.57%  (p=0.000 n=10+10)
[Geo mean]                         3.45ms         3.44ms       -0.46%

name                             old speed      new speed      delta
Encode/Digits/Huffman/1e4-6       152MB/s ± 0%   152MB/s ± 0%  +0.21%  (p=0.009 n=10+9)
Encode/Digits/Huffman/1e5-6       158MB/s ± 0%   158MB/s ± 0%    ~     (p=0.336 n=8+9)
Encode/Digits/Huffman/1e6-6       159MB/s ± 1%   159MB/s ± 1%    ~     (p=0.781 n=10+10)
Encode/Digits/Speed/1e4-6        35.6MB/s ± 0%  35.7MB/s ± 1%  +0.34%  (p=0.020 n=8+10)
Encode/Digits/Speed/1e5-6        37.6MB/s ± 0%  37.7MB/s ± 0%  +0.09%  (p=0.049 n=10+10)
Encode/Digits/Speed/1e6-6        38.0MB/s ± 0%  38.0MB/s ± 0%    ~     (p=0.146 n=10+10)
Encode/Digits/Default/1e4-6      18.0MB/s ± 0%  18.0MB/s ± 0%  -0.45%  (p=0.002 n=9+10)
Encode/Digits/Default/1e5-6      11.6MB/s ± 1%  11.6MB/s ± 1%    ~     (p=0.644 n=10+10)
Encode/Digits/Default/1e6-6      10.8MB/s ± 1%  10.8MB/s ± 1%  +0.51%  (p=0.044 n=10+10)
Encode/Digits/Compression/1e4-6  17.9MB/s ± 1%  17.9MB/s ± 1%    ~     (p=0.468 n=10+10)
Encode/Digits/Compression/1e5-6  11.7MB/s ± 0%  11.6MB/s ± 1%    ~     (p=0.322 n=8+10)
Encode/Digits/Compression/1e6-6  10.8MB/s ± 1%  10.8MB/s ± 1%    ~     (p=0.983 n=10+10)
Encode/Twain/Huffman/1e4-6        112MB/s ± 0%   112MB/s ± 1%  +0.42%  (p=0.002 n=8+9)
Encode/Twain/Huffman/1e5-6        138MB/s ± 1%   137MB/s ± 0%    ~     (p=0.616 n=10+9)
Encode/Twain/Huffman/1e6-6        139MB/s ± 0%   139MB/s ± 1%    ~     (p=0.652 n=8+10)
Encode/Twain/Speed/1e4-6         31.3MB/s ± 1%  31.2MB/s ± 1%    ~     (p=0.342 n=10+10)
Encode/Twain/Speed/1e5-6         38.0MB/s ± 0%  38.1MB/s ± 1%  +0.33%  (p=0.011 n=8+10)
Encode/Twain/Speed/1e6-6         38.8MB/s ± 0%  38.7MB/s ± 0%    ~     (p=0.325 n=10+8)
Encode/Twain/Default/1e4-6       14.8MB/s ± 1%  14.9MB/s ± 1%  +0.88%  (p=0.000 n=10+10)
Encode/Twain/Default/1e5-6       9.48MB/s ± 1%  9.68MB/s ± 0%  +2.11%  (p=0.000 n=10+10)
Encode/Twain/Default/1e6-6       8.86MB/s ± 1%  9.03MB/s ± 1%  +1.97%  (p=0.000 n=10+9)
Encode/Twain/Compression/1e4-6   14.5MB/s ± 0%  14.7MB/s ± 1%  +1.31%  (p=0.000 n=7+10)
Encode/Twain/Compression/1e5-6   8.63MB/s ± 1%  8.82MB/s ± 1%  +2.17%  (p=0.000 n=10+10)
Encode/Twain/Compression/1e6-6   7.92MB/s ± 1%  8.05MB/s ± 1%  +1.59%  (p=0.000 n=10+10)
[Geo mean]                       29.0MB/s       29.1MB/s       +0.47%

// symSizeComp `which go` go_old:

section differences:
global text (code) = 203 bytes (0.005131%)
read-only data = 1 bytes (0.000057%)
Total difference 204 bytes (0.003297%)

Change-Id: Ie2cdfa1216472d78694fff44d215b3b8e71cf7bf
Reviewed-on: https://go-review.googlesource.com/102277
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>