Made IsAssignableVisitor ignore Poison values

This fixes some spurious errors being reported by DSLParser.

Change-Id: I824896b88c2d208aac13cc8f597a2f949b831196
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/443057
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
1 file changed