[MLIR][OpenMP] Place alloca scope within wsloop in scf.parallel to omp lowering

https://reviews.llvm.org/D120423 replaced the use of stacksave/restore with memref.alloca_scope, but kept the save/restore at the same location. This PR places the allocation scope within the wsloop, thus keeping the same allocation scope as the original scf.parallel (e.g. no longer over stack allocating).

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D120772