[llvm] [SimplifyCFG] Drop UB-implying metadata when hoisting past side effects (PR #226746)

[Mian Miftah via llvm-commits]

mmiftahx wrote:

@nikic this is the SimplifyCFG fix from #225921 . The tests use `!dereferenceable` rather than `!nofreeobj`, because `combineMetadataForCSE()` still drops `!nofreeobj` on main. Call-site attributes like `dereferenceable` have the same problem when hoisting past writes. This PR doesn't change those.

https://github.com/llvm/llvm-project/pull/226746