[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,
- Don't set the insertion point in the body callback.
- 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