Migrate setRefKind assignability checker into SkSLAnalysis.

This will allow the inliner to use IsAssignable.

Change-Id: Ic94f71002779b53d0b3dc97f37fbe4bb98b026d8
Bug: skia:10756
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/319414
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
9 files changed