clean up for SSSE3 bilerp

This is mostly refactoring, moving functions to lambdas,
recommenting, that sort of thing.  Just how I relearn code...

I did pull the same trick as with SSE2 to turn one multiply
into a shift in interpolate_in_x_and_y().  Nothing interesting
beyond that.

Change-Id: I6645fa03264ec374950a5a936af05b480a69ff06
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/244321
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Klein <mtklein@google.com>
1 file changed