Implement comma operator in RP codegen.

This fixes one additional test. It also caused us to run a test
(shared/Matrices) which generated an invalid result because
Matrix * Matrix was being performed incorrectly (componentwise);
this case is now properly detected as an unsupported expression.

Change-Id: I2396d565971158648ec5bd81e7861ea5ad95404f
Bug: skia:13676
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/629436
Reviewed-by: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
2 files changed