[MLIR][OpenMP] Simplify OpenMP device codegen (#137201)
After removing host operations from the device MLIR module, it is no
longer necessary to provide special codegen logic to prevent these
operations from causing compiler crashes or miscompilations.
This patch removes these now unnecessary code paths to simplify codegen
logic. Some MLIR tests are now replaced with Flang tests, since the
responsibility of dealing with host operations has been moved earlier in
the compilation flow.
MLIR tests holding target device modules are updated to no longer
include now unsupported host operations.
