Lowering of OpenMP Parallel operation to LLVM IR 1/n

This patch introduces lowering of the OpenMP parallel operation to LLVM
IR using the OpenMPIRBuilder.

Functions topologicalSort and connectPhiNodes are generalised so that
they work with operations also. connectPhiNodes is also made static.

Lowering works for a parallel region with multiple blocks. Clauses and
arguments of the OpenMP operation are not handled.

Reviewed By: rriddle, anchu-rajendran

Differential Revision: https://reviews.llvm.org/D81660