Focus disabled optimizations on just BGRAConvolve2D, and only 32-bit.

Last CL accidentally disabled autovectorization on 64-bit builds too.
This fixes that.

BUG=skia:2575
R=rmistry@google.com, mtklein@google.com

Author: mtklein@chromium.org

Review URL: https://codereview.chromium.org/353823005
2 files changed