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

[Vyacheslav Levytskyy via llvm-commits]

VyacheslavLevytskyy 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.
> 
> I will check now. For one the reason is clear, #123191

I can't reproduce two other fails on different environments, including actually Github Actions, so it maybe that after you rebase to `main` we will see all green now.

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