Rename SkNi to SkNb.

Rename SkNi to SkNb.

As used today, SkNi is used in bool-y contexts.  This keeps that, but under a
new name, SkNb.  This makes room for a new SkNi that's focused on integer-y
things like loads, stores, arithmetic, etc.

The main reason to split these is that we want different specializations for
each use case: for bools, it's important for us to specialize 32- and 64-bit to
support efficient float- and double- comparisons, but for integer work we're
more likely to be looking at 8- and 16- bit lanes.  Keeping these use cases
siloed helps me manage the compexity of the backend NEON and SSE code.

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/8fe8fffdfa7464c6f7da773b8660a2043f4998e0

[mtklein, 2015-04-14 18:32:15]

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

[mtklein_C, 2015-04-14 18:32:29]

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

[commit-bot: I haz the power, 2015-04-14 18:32:29]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1083123002/20001

[commit-bot: I haz the power, 2015-04-14 18:39:09]

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

[commit-bot: I haz the power, 2015-04-14 18:39:09]

This issue passed the CQ dry run.

[reed1, 2015-04-14 18:48:20]

lgtm

[mtklein, 2015-04-14 18:48:40]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2015-04-14 18:49:03]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1083123002/20001

[commit-bot: I haz the power, 2015-04-14 18:49:19]

Committed patchset #2 (id:20001) as
https://skia.googlesource.com/skia/+/8fe8fffdfa7464c6f7da773b8660a2043f4998e0