Implement constantOutputForConstantInput on GrSkSLFP

Bug: skia:11770
Change-Id: I118a85e58a5422ff42c693fd8f85af098d8f8a25
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/387816
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
5 files changed