[graphite] Apply a few simplifications in Graphite shaders.

This CL flattens out a few cases where comparisons can be reduced
to branchless logic. It shouldn't change results.

Change-Id: I7064f737de5df0126de36103a7b375eafd574209
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/855417
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
3 files changed