[llvm] [SPIR-V] Fix an undefined behaviour in the SPIRV emit-intrinsics pass (PR #123620)

[Vyacheslav Levytskyy via llvm-commits]

VyacheslavLevytskyy wrote:

> I started working on a similar change yesterday evening, but refactored getting the instruction set into a separate function and removed some repetition in another place: #123625

Yes, it's even better, thank you

https://github.com/llvm/llvm-project/pull/123620