[llvm] [SPIR-V] Rework usage of virtual registers' types and classes (PR #101732)

[Farzon Lotfi via llvm-commits]

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

LGTM, but I'm mostly checking for HLSL use cases and don't have experience with Extensions. I would wait for a more experienced reviewer before merging. 

https://github.com/llvm/llvm-project/pull/101732