SkSL interpreter optimizations

Bug: skia:
Change-Id: If1e4916fc40fa5799dabd446e61b840ba3a32133
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/211989
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: Mike Reed <reed@google.com>
4 files changed