added SkSL OptimizationContext

This simplifies the signatures of several methods and makes it easier to
ignore errors we have already reported.

Change-Id: I767ca22a66e69fe72557c2560ef84f5dbe339eb8
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/329220
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>

Author: Ethan Nicholas