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

[Steven Perron via llvm-commits]

https://github.com/s-perron commented:

I like the algorithm. It is simple and clean. However, I think the implementation can be trimmed a bit. 

The main thing is the testing. There are many cases we need to consider, to make sure the right capabilities are added. Please add tests that would correspond to an array of cbuffer, buffers, RWBuffers, structuredBuffers, and RWStructruedBuffers. Each of those require different capabilities.

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