MSL: Fragment shader that discards unconditionally does not compile.

[squidbus]

Ran into an interesting case with a emulator shader translator where it was outputting a shader that branches over its whole logic to a OpDemoteToHelperInvocation at the end.

The SPIR-V at the end looks like this:

       %2613 = OpLabel
               OpDemoteToHelperInvocation
               OpBranch %195
        %195 = OpLabel
               OpUnreachable
               OpFunctionEnd

When translated to MSL, the shader results in a main function with the following structure:

fragment main0_out main0(main0_in in [[stage_in]], ...)
{
    main0_out out = {};
    ...
    if (false)
    {
        ...
        return out;
    }
    discard_fragment();
}

This results in the following MSL compile error:

program_source:1199:1: error: control reaches end of non-void fragment function [-Werror,-Wmtl-shader-return-type]
}
^

Now, I am not sure whether this is actually how the shader was supposed to be translated from its original assembly to SPIR-V. I've looked into this further and the shader does in fact just block its logic behind an always-false condition and discard. I do believe the resulting SPIR-V is a valid shader with no errors from spirv-val, so I think cases like this where a shader discards unconditionally should produce valid MSL shaders. I believe adding a dummy return after the discard can fix the issue, see another case of this for reference: gfx-rs/wgpu#4458

[squidbus]

I saw there's similar handling for blocks ending in Kill already and it just needs to be extended to Unreachable, so I added that here: #2436

[HansKristian-Work]

       %2613 = OpLabel
               OpDemoteToHelperInvocation
               OpBranch %195
        %195 = OpLabel
               OpUnreachable
               OpFunctionEnd

It should be mentioned that this SPIR-V is broken, but we can paper over it.