[clang] [llvm] [SPIRV] add pre legalization instruction combine (PR #122839)

[Farzon Lotfi via cfe-commits]

farzonl wrote:

> I see also some problems in test cases

@VyacheslavLevytskyy the SPIRV test case failures are not related to my change. They are also failing on main.

https://github.com/llvm/llvm-project/pull/122839