[Flang][OpenMP] Run Flang-specific OpenMP MLIR passes in bbc

This patch moves the group of OpenMP MLIR passes using after lowering of
Fortran to MLIR into a pipeline to be shared by `flang-new` and `bbc`.
Currently, the `bbc` tool does not produce the expected FIR for offloading-
enabled OpenMP codes due to not running these passes.

Unit tests exercising these passes are updated to check `bbc` output as well.