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:

Review URL: https://codereview.chromium.org/1083123002
4 files changed