trifurcate blit_mask_d32_a8 into _black, _opaque, _general.

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
Committed: https://skia.googlesource.com/skia/+/5015176adf046ef906a2313b6e6b64b72cc84898

[mtklein_C, 2015-08-26 16:58:08]

The CQ bit was checked by mtklein@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-08-26 16:58:21]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1304373006/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1304373006/1

[commit-bot: I haz the power, 2015-08-26 17:06:48]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-08-26 17:06:49]

Dry run: This issue passed the CQ dry run.

[mtklein_C, 2015-08-26 17:29:16]

mtklein@chromium.org changed reviewers:
+ reed@google.com

[reed1, 2015-08-26 19:14:16]

if its faster, it seems easy enough to read/maintain

lgtm

[mtklein_C, 2015-08-26 19:34:24]

On 2015/08/26 19:14:16, reed1 wrote:
> if its faster, it seems easy enough to read/maintain
> 
> lgtm

I think we're going to see the effect most clearly on text_16_AA_BK:
https://perf.skia.org/#3980

Locally I'm getting a ~20% speedup there.

[mtklein_C, 2015-08-26 19:34:32]

The CQ bit was checked by mtklein@chromium.org

[commit-bot: I haz the power, 2015-08-26 19:34:39]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1304373006/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1304373006/1

[commit-bot: I haz the power, 2015-08-26 19:35:17]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/5015176adf046ef906a2313b6e6b64b72cc84898