SkBlurImageFilter_opts: optimize NEON box_blur_double in separate loops.

Stop leaning so hard on the branch predictor, and pull the conditionals
out of the loops for box_blur_double() (NEON).  This is conceptually
the same change as https://codereview.chromium.org/1426583004/ for
the NEON double-pixel loop.

R=mtklein@google.com

BUG=skia:4526
CQ_EXTRA_TRYBOTS=client.skia:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD-Trybot

Review URL: https://codereview.chromium.org/1412793009
1 file changed