[llvm] [SPIR-V] Support `nonuniformindex` intrsinsic in SPIRV CodeGen. (PR #162540)

[Lucie Choi via llvm-commits]

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luciechoi wrote:

Thanks for pointing out! 

Based on our discussion
- Separated the tests for `RWStructuredBuffer` and `RWBuffer`.
- Other resource types https://godbolt.org/z/johhnWse1 and https://godbolt.org/z/Gr8oKjxMb aren't yet added to the frontend or have bugs in their implementation, will open up the issues!

https://github.com/llvm/llvm-project/pull/162540