DepthGreater outside of Fragment not caught by spirv-val

[zoddicus]

White writing tests for a different PR, I wrote a parameterized test that put in a bunch of different execution modes for a GLCompute shader. The validator correctly caught the other Depth* modes being Fragment only, but it didn't error on DepthGreater. Checking the spec, I am pretty sure this should have thrown the same type of error, so I am going to investigate if this case was missed.