Add peephole optimization for merge_condition_mask.

This is not a huge win, but easy to do now that current-stack index
is baked into the Instruction instead of handled as a BuilderOp.

Change-Id: Ie692ed97dd90130f100152150f2778214e1cad9a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/716547
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
5 files changed