Specialize Sk2d for SSE2

Given the autovectorization we've seen, I wouldn't expect big speedups
from this, but it does give us a point of control over what's going on.

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

Review URL: https://codereview.chromium.org/1526923003
2 files changed