Detect alpha-preservation in ternary expressions.

It occurred to me that this was fairly easy to check. I'm not sure if
any of our shaders would actually exercise this path.

Change-Id: Iab22688b00a2d55333717f4ea9a0d92cc8e5e756
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/665857
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
2 files changed