Add GetConstantValueOrNullForVariable to constant folder.

Change-Id: I906e9f52a75373a96edc1cb399cdc0dc5c729d6b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/598976
Reviewed-by: Arman Uguray <armansito@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: Arman Uguray <armansito@google.com>
3 files changed