Move blend fallback case to default case.

This allows the optimizer to remove it when the switch value is known,
instead of waiting for the dead-code-elimination pass to find it.
(If we decide to remove the dead-code-elimination pass entirely, this
will generate better code overall.)

Change-Id: I79a40eb3cfbc7e5d2f4879e1f98323248446483b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/373876
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
2 files changed