Checks if early fragment tests as an optimization is feasible and enable
it if we can. In the link time, if context state diagrees with
optimization (in rare case), then remove the
ExecutionModeEarlyFragmentTests sprv op code.

Bug: angleproject:4508
Change-Id: Ifbb06c0ffb050a9f3ddb16ab50362e908b4b9cf6
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2136490
Commit-Queue: Charlie Lao <cclao@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>