Simplify SkRP pop-condition-mask followed by a matching push.

The push can be dropped, since the condition mask is already on the
stack at the desired location.

Change-Id: I020beadf3a4b4788dfa2652fb69ad584e52d33b0
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/797822
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
6 files changed