Determine correct behaviour for derivative in compute shaders

[s-perron]

The validator does not allow derivative instructions in compute shader unless the entry point has either the DerivativeGroupQuadsNV or DerivativeGroupLinearNV execution mode. However, the specification for the extension says:

If neither derivative group mode was specified, the derivatives return zero.

See it here.

Here is a test that checks for the incorrect behaviour.

SPIRV-Tools/test/val/val_derivatives_test.cpp

Lines 163 to 178 in 661f429

	TEST_F(ValidateDerivatives, NoExecutionModeGLCompute) {
	const std::string spirv = R"(
	OpCapability Shader
	OpMemoryModel Logical GLSL450
	OpEntryPoint GLCompute %main "main"
	%void = OpTypeVoid
	%float = OpTypeFloat 32
	%float4 = OpTypeVector %float 4
	%undef = OpUndef %float4
	%void_fn = OpTypeFunction %void
	%main = OpFunction %void None %void_fn
	%entry = OpLabel
	%derivative = OpDPdy %float4 %undef
	OpReturn
	OpFunctionEnd
	)";

If the validator behaviour is wrong, then the changes in #5430 will have to be updated as well.

[s-perron]

FYI: @dgkoch @jmacnak-nv.

[s-perron]

This issue was being discussed in the PR where it was first observed: #5430 (comment)