Determine correct behaviour for derivative in compute shaders

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](https://htmlpreview.github.io/?https://github.com/KhronosGroup/SPIRV-Registry/blob/master/extensions/NV/SPV_NV_compute_shader_derivatives.html#:~:text=If%20neither%20derivative%20group%20mode%20was%20specified%2C%20the%20derivatives%20return%20zero.).

Here is a test that checks for the incorrect behaviour.

https://github.com/KhronosGroup/SPIRV-Tools/blob/661f429b11e4392139a6c0630ceb3e3182cdb0f4/test/val/val_derivatives_test.cpp#L163-L178

If the validator behaviour is wrong, then the changes in https://github.com/KhronosGroup/SPIRV-Tools/pull/5430 will have to be updated as well.