[mlir] Refactor translation of OpenMP dialect ops to LLVM IR

The original implementation of the OpenMP dialect to LLVM IR translation has
been relying on a stack of insertion points for delayed insertion of branch
instructions that correspond to terminator ops. This is an intrusive into
ModuleTranslation and makes the translation non-local. A recent addition of the
WsLoop translation exercised another approach where the parent op is
responsible for converting terminators of all blocks in its regions. Use this
approach for other OpenMP dialect operations with regions, remove the stack and
deduplicate the code for converting such regions.

Reviewed By: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D94086