[clang] [llvm] [HLSL] Implement ddx/ddy_coarse intrinsics (PR #164831)

[Alexander Johnston via llvm-commits]

Alexander-Johnston wrote:

@farzonl I've updated this and should have fixed the test failures. I'd place emphasis on the SPIRV instruction selection changes I've made. SPIRV only allows float as an input to the `OpDPd*` while HLSL takes half or float, so I've wrapped it in `OpFConvert` to switch to and from.
As part of this I've fixed what I think is a bug I caught in the SPIRVGlobalRegistry that unintentionally added FP Encoding 0 (BFloat16KHR) to any `OpFloatTy` created by `getOrCreateSPIRVFloatType`.

https://github.com/llvm/llvm-project/pull/164831