[SPIR-V] Crash in `addDerivativeGroupExecutionMode()` when called in Vertex Shader

[LukasBanana]

We have a crash in UE5 which looks to me like a result from a cherrypicked change (#7445) that adopted the condition !spvContext.isPS() instead of spvContext.isCS() which all other locations use before calling addDerivativeGroupExecutionMode():

DirectXShaderCompiler/tools/clang/lib/SPIRV/SpirvEmitter.cpp

Line 9613 in d72e2b1

if (!spvContext.isPS())

This function even has an assertion for this condition:

assert(spvContext.isCS());

Interestingly, though, we hit a null pointer crash instead of this assertion.

Just wanted to raise this here because I won't have time for a PR including unit tests due to our release schedule.

[s-perron]

Bad testing. This is because with the KHR extension, you can add the derivative instructions in more than just CS shaders, and you need the derivative group execution mode for those other ones as well.

It seems like DXC allows using the derivative instruction in VS as long as they get optimized away. I'll try to get more through fix for this.

• Modify addDerivativeGroupExecutionMode to only add the exe mode for compute, mesh, and amplification shaders. Return doing nothing otherwise. We may want to check if we are using the NV or KHR extension to determine what we do.
• Extend this to implicit derivative instructions as well.
• Add tests for mesh and amplification shaders with NV and KHR extensions.