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

[Farzon Lotfi via llvm-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.
> > 
> > 
> > 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.

I rebased and the tests are passing locally. but they are still failing in ci.
```bash
bash-3.2$ ../llvm_debug_build/bin/llvm-lit -sv llvm/test/CodeGen/SPIRV/branching/OpSwitchUnreachable.ll

Testing Time: 2.29s

Total Discovered Tests: 1
  Passed: 1 (100.00%)
bash-3.2$ ../llvm_debug_build/bin/llvm-lit -sv llvm/test/CodeGen/SPIRV/keep-tracked-const.ll

Testing Time: 0.21s

Total Discovered Tests: 1
  Passed: 1 (100.00%)
```

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