[llvm] [DirectX][DXIL] Design document for TableGen Spec of DXIL Operations (PR #85170)

[Justin Bogner via llvm-commits]

https://github.com/bogner approved this pull request.

As Chris mentioned in https://github.com/llvm/llvm-project/pull/85170#discussion_r1618007508, there are a few glaring gaps here that I think will need to be resolved in order to generate correct DXIL:

1. Derivatives, which became valid in more shader stages in SM6.6
2. Atomics, which gained 64 bit overloads in SM6.6
3. Sample ops gained [integer overloads](https://microsoft.github.io/DirectX-Specs/d3d/HLSL_SM_6_7_Advanced_Texture_Ops.html#integer-sampling) in SM6.7
4. Sample and Texture ops may be affected by SM6.7 [programmable offsets](https://microsoft.github.io/DirectX-Specs/d3d/HLSL_SM_6_7_Advanced_Texture_Ops.html#programmable-offsets)

It isn't ideal to move forward with this without at least having an idea of how we'll handle those scenarios. However, it's probably sensible to get this in tree and iterate on it there as the design evolves.

https://github.com/llvm/llvm-project/pull/85170