[MLIR,OpenMP] Added support for lowering MasterOp to LLVMIR

Some Ops in OMP dialect have regions associated with them i.e
`ParallelOp` `MasterOp`. Lowering of these regions involves interfacing
with `OMPIRBuilder` using callbacks, yet there still exist opportunities
for sharing common code in between.

This patch factors out common code into a separate function and adds
support for lowering `MasterOp` using that. Lowering of `ParallelOp` is
also modified appropriately.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D87247