Fix bug in GrClearOp combining and remove some asserts (take 2)

The buffer combining code path was combining the ops but never
telling the external system that the second op could be removed.

Bug: skia:10963
Change-Id: I887ebda91673d37139ebc7fc427e80a55b9d9bd1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/340101
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
3 files changed