Fix assertion in constant-folder when using float4(mat2) casts.

The no-op-arithmetic simplifier was written before we allowed casting a
mat2x2 to a float4, and did not expect a matrix inside a vector ctor.
The expression `float4(myMat2) * float4(anything)` would assert when we
tried to determine if `myMat2` was a constant zero or constant one.

The code has been rewritten to use getConstantSubexpression and now
allows matrices inside.

Change-Id: Id625141256bf89d816c57d2d21f16b0ec252c158
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/440858
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
3 files changed