[SPIRV] The result of `isShader` changes in the middle of compilation

[s-perron]

In #129689, a new function SPIRVSubtarget::isShader was added. It is suppose to tell us of the module is a shader module so that the spir-v can be generated accordingly. The problem is that sometime, the compiler cannot know if the module is a shader by looking at the triple alone, so code was added to change the environment in the subtarget when lowering a function with the "hlsl.shader" attribute.

llvm-project/llvm/lib/Target/SPIRV/SPIRVCallLowering.cpp

Lines 256 to 264 in 854ef8d

	// We will now change the Env based on the attribute, so we need to strip
	// `const` out of the ref to STI.
	SPIRVSubtarget *NonConstSTI = const_cast<SPIRVSubtarget *>(&STI);
	auto attribute = F.getFnAttribute("hlsl.shader");
	if (!attribute.isValid()) {
	NonConstSTI->setEnv(SPIRVSubtarget::Kernel);
	return SPIRV::ExecutionModel::Kernel;
	}
	NonConstSTI->setEnv(SPIRVSubtarget::Shader);

This means that if isShader is called before the entry point is processed, it will return false, and code that is not valid for a shader may be generated. This value needs to be set at the start of the compilation, and should not change.