Port ARGBExtractAlpha_AVX2 function to windows.

Port ARGBExtractAlpha_AVX2 function to windows.

BUG=libyuv:572
TEST=try bots
R=magjed@chromium.org, wangcheng@google.com

[fbarchard1, 2016-10-13 23:28:40]

Tested locally with emulator

c:\intelsde\sde -mix -omix mix.txt -ast -oast ast.txt -skx --
out\release\libyuv_unittest.exe --gtest_cat
ch_exceptions=0 --gtest_filter=*ExtractAlpha*
Note: Google Test filter = *ExtractAlpha*
[==========] Running 1 test from 1 test case.
[----------] Global test environment set-up.
[----------] 1 test from LibYUVPlanarTest
[ RUN      ] LibYUVPlanarTest.TestARGBExtractAlpha
[       OK ] LibYUVPlanarTest.TestARGBExtractAlpha (126 ms)
[----------] 1 test from LibYUVPlanarTest (201 ms total)

[----------] Global test environment tear-down
[==========] 1 test from 1 test case ran. (562 ms total)
[  PASSED  ] 1 test.

[wangcheng, 2016-10-14 00:46:10]

lgtm

[fbarchard1, 2016-10-14 06:12:38]

C on macoS
TestARGBExtractAlpha (9162 ms)

SSE2
TestARGBExtractAlpha (4410 ms)

AVX2
TestARGBExtractAlpha (3523 ms)

[fbarchard1, 2016-10-14 18:22:05]

Description was changed from

==========
Port ARGBExtractAlpha_AVX2 function to windows.

BUG=libyuv:572
TEST=try bots
R=magjed@chromium.org, wangcheng@google.com
==========

to

==========
Port ARGBExtractAlpha_AVX2 function to windows.

BUG=libyuv:572
TEST=try bots
R=magjed@chromium.org, wangcheng@google.com
==========

[fbarchard1, 2016-10-14 18:22:13]

commit 5333e94e70fffcb126fe507cabbaefa0783b8125