Implement four more xfermodes with Sk4px.

Implement four more xfermodes with Sk4px.

HardLight, Overlay, Darken, and Lighten are all
~2x faster with SSE, ~25% faster with NEON.

This covers all previously-implemented NEON xfermodes.
3 previous SSE xfermodes remain.  Those need division
and sqrt, so I'm planning on using SkPMFloat for them.
It'll help the readability and NEON speed if I move that 
into [0,1] space first.

The main new concept here is c.thenElse(t,e), which behaves like
(c ? t : e) except, of course, both t and e are evaluated.  This allows
us to emulate conditionals with vectors.

This also removes the concept of SkNb.  Instead of a standalone bool
vector, each SkNi or SkNf will just return their own types for
comparisons.  Turns out to be a lot more manageable this way.

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/b9d4163bebab0f5639f9c5928bb5fc15f472dddc

CQ_EXTRA_TRYBOTS=client.skia.compile:Build-Ubuntu-GCC-Arm64-Debug-Android-Trybot

Committed: https://skia.googlesource.com/skia/+/b5e861185a69b3e47d1c1bf622fd3f83e5f13898

[mtklein, 2015-06-24 19:08:04]

The CQ bit was checked by mtklein@google.com to run a CQ dry run

[commit-bot: I haz the power, 2015-06-24 19:08:26]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1196713004/230001

[commit-bot: I haz the power, 2015-06-24 19:11:46]

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

[commit-bot: I haz the power, 2015-06-24 19:11:48]

Dry run: Try jobs failed on following builders:
  Build-Win-MSVC-x86-Debug-Trybot on client.skia.compile (JOB_FAILED,
http://build.chromium.org/p/client.skia.compile/builders/Build-Win-MSVC-x86-D...)
  Build-Win-MSVC-x86_64-Debug-Trybot on client.skia.compile (JOB_FAILED,
http://build.chromium.org/p/client.skia.compile/builders/Build-Win-MSVC-x86_6...)

[mtklein, 2015-06-24 19:11:59]

The CQ bit was checked by mtklein@google.com to run a CQ dry run

[commit-bot: I haz the power, 2015-06-24 19:12:19]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1196713004/250001

[commit-bot: I haz the power, 2015-06-24 19:15:31]

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

[commit-bot: I haz the power, 2015-06-24 19:15:32]

Dry run: Try jobs failed on following builders:
  Build-Win-MSVC-x86-Debug-Trybot on client.skia.compile (JOB_FAILED,
http://build.chromium.org/p/client.skia.compile/builders/Build-Win-MSVC-x86-D...)
  Build-Win-MSVC-x86_64-Debug-Trybot on client.skia.compile (JOB_FAILED,
http://build.chromium.org/p/client.skia.compile/builders/Build-Win-MSVC-x86_6...)

[mtklein, 2015-06-24 19:20:20]

The CQ bit was checked by mtklein@google.com to run a CQ dry run

[commit-bot: I haz the power, 2015-06-24 19:20:36]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1196713004/270001

[commit-bot: I haz the power, 2015-06-24 19:26:23]

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

[commit-bot: I haz the power, 2015-06-24 19:26:24]

Dry run: This issue passed the CQ dry run.

[mtklein_C, 2015-06-24 19:27:53]

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

[mtklein_C, 2015-06-24 19:57:44]

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

[commit-bot: I haz the power, 2015-06-24 19:57:58]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1196713004/290001

[commit-bot: I haz the power, 2015-06-24 20:03:40]

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

[commit-bot: I haz the power, 2015-06-24 20:03:42]

Dry run: This issue passed the CQ dry run.

[reed1, 2015-06-24 21:26:04]

lgtm

[mtklein, 2015-06-24 21:26:48]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2015-06-24 21:27:06]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1196713004/290001

[commit-bot: I haz the power, 2015-06-24 21:27:40]

Committed patchset #16 (id:290001) as
https://skia.googlesource.com/skia/+/b9d4163bebab0f5639f9c5928bb5fc15f472dddc

[mtklein, 2015-06-24 21:30:18]

A revert of this CL (patchset #16 id:290001) has been created in
https://codereview.chromium.org/1205703008/ by mtklein@google.com.

The reason for reverting is: 64-bit ARM build failures..

[mtklein, 2015-06-24 22:01:51]

The CQ bit was checked by mtklein@google.com

[mtklein, 2015-06-24 22:01:51]

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/1196713004/#ps310001
(title: "manually cast for missing 64-bit vreinterprets")

[commit-bot: I haz the power, 2015-06-24 22:01:59]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1196713004/310001

[commit-bot: I haz the power, 2015-06-24 22:18:43]

Committed patchset #17 (id:310001) as
https://skia.googlesource.com/skia/+/b5e861185a69b3e47d1c1bf622fd3f83e5f13898