Remove fExpression (matrix expression) from SampleUsage

Other than GMs, there is exactly one use of matrix-sampling
(GrMatrixEffect). It is always uniform (not literal or an
expression containing a uniform). The uniform always has the
same name. Bake these simplifications in, which also shrinks
SampleUsage quite a bit.

Change-Id: I0d5e32069d710af475ccc1030e2988c5fc965a98
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/411296
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
9 files changed