comment and TODO bilerp code

I'm starting my way towards AVX2 code here, first starting with the
relatively simple SSE2 code to make sure I understand what's going on.

Renames, updated comments, and one performance-related TODO.

Change-Id: I366ee67cf89c8c8d2ccc610d6d753531a131e7b9
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/243815
Commit-Queue: Mike Klein <mtklein@google.com>
Auto-Submit: Mike Klein <mtklein@google.com>
Reviewed-by: Mike Reed <reed@google.com>
1 file changed