Replace getMatExpression with getConstantSubexpression.

This approach gives us similar flexibility but requires fewer lines of
code to get the same result. In a followup CL we will be able to
eliminate get[BFI]VecExpression as well. This approach will also scale
to arrays and structs if we want to support constant-folding on these.

Change-Id: Ib0034935926c7004f84ba62ddbdb3168df8ce91d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/393076
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
14 files changed