[OpenMP][MLIR] Fix for nested parallel regions

Usage of nested parallel regions were not working correctly and leading
to assertion failures. Fix contains the following changes,

1. Don't set the insertion point in the body callback.
2. Save the continuation IP in a stack and set the branch to

continuationIP at the terminator.

Reviewed By: SouraVX, jdoerfert, ftynse

Differential Revision: https://reviews.llvm.org/D88720