fix clamp_0/clamp_1 stages in lowp

The existing strategy for skipping them never worked, and the new test
would have crashed.  Instead just make them normal but trivial stages.

Change-Id: I41772e817e52f585b2b90680fdd1781025caf2d1
Reviewed-on: https://skia-review.googlesource.com/148981
Commit-Queue: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Auto-Submit: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
3 files changed