[clang] [llvm] [SPIRV] Add PreLegalizer instCombine for `faceforward` (PR #139959)

[Steven Perron via cfe-commits]

https://github.com/s-perron commented:

I have a minor concern about how specific the `N` and `0-N` checks are. Otherwise this looks good to me.

https://github.com/llvm/llvm-project/pull/139959