constexprify GrBlend.h and GrPorterDuffXferProcessor.cpp

Also remove some unused functions from GrBlend.h and related unit test.

Bug: skia:
Change-Id: Id8ad0057a02f65a9e19dc75e4b88709a762f4139
Reviewed-on: https://skia-review.googlesource.com/12623
Reviewed-by: Chris Dalton <csmartdalton@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
4 files changed