Fix Plus
Fix Plus

This makes Plus with AA ~3% slower.  Seems like a good deal.

GMs affected: mixed_xfermodes, the new one.

Based on https://codereview.chromium.org/1150833003/

Still TODO: NEON.  The new GM should show this.

BUG=skia:3852

Review URL: https://codereview.chromium.org/1156453002

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