[mlir][llvm][OpenMP] Hoist __atomic_load alloca (#132888)
Current implementation of `__atomic_compare_exchange` uses an alloca for
`__atomic_load`, leading to issues like
https://github.com/llvm/llvm-project/issues/120724. This PR hoists this
alloca to `AllocaIP`.
Fixes: https://github.com/llvm/llvm-project/issues/120724
