Enforce fragment shader execution when operations with side effects are present before discard

[aitor-lunarg]

Failing CTS tests:

dEQP-VK.fragment_operations.early_fragment.discard_no_early_fragment_tests_depth

Test has a fragment shader with side effects and a discard operation after. However, fragments get prematurely discarded under certain circumstances as an optimization (when they shouldn't).

We can circumvent this issue by making the discard operation "optional" whenever a side effect operation is present before the discard. Equivalent Metal code for the following GLSL: if (!gl_HelperInvocation) discard;

So, similar to #2232 I believe this is an oversight from Metal's end. MoltenVK should provide a short term solution for this case, and hopefully Metal fixes this issue in the long term.