trifurcate blit_mask_d32_a8 into _black, _opaque, _general.

We used to split the NEON code this way, and just had one path for SSE.
It's unclear to me testing locally if there's any major win here, but there's at least a small one.

No pixel diffs or even any math changes, just folding constants through.

BUG=skia:4117

Review URL: https://codereview.chromium.org/1304373006
1 file changed