[llvm] [SPIRV][HLSL] Add FixedVector GEP legalization (PR #171682)

[Steven Perron via llvm-commits]

https://github.com/s-perron approved this pull request.

I'm okay with this change. The original comment in the code was trying to align with [this discussion](https://discourse.llvm.org/t/status-of-geps-into-vectors-of-overaligned-elements/67497/11) on discourse. However, it is unclear how far that is going.

We can try to handle the cases that come up. I don't think we should make the SPIR-V backend special. If the element size if a multiple of 8, which it will always be coming from HLSL, we can properly represent the GEP with an access chain.

As discussed in the HLSL working group meeting, the entire function being modified is just the short term solution while we get the structured GEP in place.

https://github.com/llvm/llvm-project/pull/171682