3-15% speedup to HardLight / Overlay xfermodes.

3-15% speedup to HardLight / Overlay xfermodes.

While investigating my bug (skia:4052) I saw this TODO and figured
it'd make me feel better about an otherwise unsuccessful investigation.

This speeds up HardLight and Overlay (same code) by about 15% with SSE, mostly
by rewriting the logic from 1 cheap comparison and 2 expensive div255() calls
to 2 cheap comparisons and 1 expensive div255().

NEON speeds up by a more modest ~3%.

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/4be181e304d2b280c6801bd13369cfba236d1a66

[mtklein_C, 2015-07-13 21:50:22]

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

[commit-bot: I haz the power, 2015-07-13 21:50:44]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1230663005/20001

[mtklein_C, 2015-07-13 22:21:55]

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

[mtklein_C, 2015-07-13 22:21:59]

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

[commit-bot: I haz the power, 2015-07-13 22:22:17]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1230663005/60001

[commit-bot: I haz the power, 2015-07-13 22:30:43]

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

[commit-bot: I haz the power, 2015-07-13 22:30:44]

Dry run: This issue passed the CQ dry run.

[reed1, 2015-07-14 13:55:36]

add a unittest?

I presume widenLoHi == widenLo | widenHi  ?

lgtm

[mtklein_C, 2015-07-14 14:34:03]

On 2015/07/14 13:55:36, reed1 wrote:
> add a unittest?
Done.

> I presume widenLoHi == widenLo | widenHi  ?
Yep.
 
> lgtm

[mtklein_C, 2015-07-14 14:34:37]

The CQ bit was checked by mtklein@chromium.org

[mtklein_C, 2015-07-14 14:34:38]

The patchset sent to the CQ was uploaded after l-g-t-m from reed@google.com
Link to the patchset: https://codereview.chromium.org/1230663005/#ps80001
(title: "Add a test that widenLo() | widenHi() == widenLoHi()")

[commit-bot: I haz the power, 2015-07-14 14:34:48]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1230663005/80001

[commit-bot: I haz the power, 2015-07-14 17:54:23]

Committed patchset #5 (id:80001) as
https://skia.googlesource.com/skia/+/4be181e304d2b280c6801bd13369cfba236d1a66