Implement compile-time optimization for pow().

Change-Id: I1630cf6ecab6a1a18a7318c247f8263d87e2cda9
Bug: skia:12034
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/412662
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
5 files changed