broadcom/compiler: don't enable early fragment tests if shader writes Z

We had an optimization to auto-enable early fragment tests when a shader didn't have side effects, but of course, we cannot do that this if the shader writes Z, as in that case the fragment tests need to use the value written from the shader.

Also, if the shader enables early fragment tests, then any shader Z writes should be ignored.

Fixes: dEQP-VK.spirv_assembly.instruction.graphics.early_fragment.*