Partial reapply of r5364 minus the non-neon code path.

See https://codereview.appspot.com/6465075 for a more detailed description of the contents of this CL.

Review URL: https://codereview.chromium.org/13060004

git-svn-id: http://skia.googlecode.com/svn/trunk@8579 2bbb7eff-a529-9590-31e7-b0007b416f81