The optimization should check of "alpha to coverage" is enabled, not
just sample coverage.

Bug: angleproject:4508
Bug: angleproject:5045
Bug: b/168493024
Change-Id: I0e52ddd4c92f9a6ea8a0e7cf56db71ce9ff3c5de
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2422078Reviewed-by: Charlie Lao <cclao@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>