[mlir] OpenMP-to-LLVM: properly set outer alloca insertion point

Previously, the OpenMP to LLVM IR conversion was setting the alloca insertion
point to the same position as the main compuation when converting OpenMP
`parallel` operations. This is problematic if, for example, the `parallel`
operation is placed inside a loop and would keep allocating on stack on each
iteration leading to stack overflow.

Reviewed By: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D101307