Introduce gimple_omp_parallel

Introduce gimple_omp_parallel

This corresponds to:
  [PATCH 39/89] Introduce gimple_omp_parallel
  https://gcc.gnu.org/ml/gcc-patches/2014-04/msg01159.html
from the original 89-patch kit

That earlier patch was approved by Jeff:
> OK with expected changes due to renaming/updates to const handling.
> Please repost the final patch for archival purposes.
in https://gcc.gnu.org/ml/gcc-patches/2014-05/msg00827.html

gcc/
	* coretypes.h (gimple_omp_parallel): New typedef.
	(const_gimple_omp_parallel): New typedef.

	* cgraphbuild.c (build_cgraph_edges): Convert check of code
	against GIMPLE_OMP_PARALLEL to a dyn_cast <gimple_omp_parallel>
	and new local.

	* gimple-pretty-print.c (dump_gimple_omp_parallel): Require a
	gimple_omp_parallel rather than a plain gimple.
	(pp_gimple_stmt_1): Add a checked cast to gimple_omp_parallel
	within GIMPLE_OMP_PARALLEL case of switch statement.

	* gimple-walk.c (walk_gimple_op): Likewise, introducing a local.

	* gimple.c (gimple_build_omp_parallel): Return a
	gimple_omp_parallel rather than a plain gimple.
	(gimple_copy): Add checked casts to gimple_omp_parallel within
	GIMPLE_OMP_PARALLEL case of switch statement, introducing locals.

	* gimple.h (gimple_build_omp_parallel): Return a
	gimple_omp_parallel rather than a plain gimple.
	(gimple_omp_parallel_clauses_ptr): Require a gimple_omp_parallel
	rather than a plain gimple.
	(gimple_omp_parallel_set_clauses): Likewise.
	(gimple_omp_parallel_data_arg_ptr): Likewise.
	(gimple_omp_parallel_set_data_arg): Likewise.
	(gimple_omp_parallel_child_fn_ptr): Likewise.
	(gimple_omp_parallel_set_child_fn): Likewise.
	(gimple_omp_parallel_child_fn): Require a
	const_gimple_omp_parallel rather than a plain const_gimple.
	(gimple_omp_parallel_data_arg): Likewise.

	* omp-low.c (scan_omp_parallel): Strengthen local "stmt" from
	gimple to gimple_omp_parallel.
	(expand_parallel_call): Require a gimple_omp_parallel for
	"entry_stmt" rather than a plain gimple.
	(remove_exit_barrier):  Strengthen local "parallel_stmt" from
	gimple to gimple_omp_parallel.
	(expand_omp_taskreg): Add checked casts to gimple_omp_parallel.

	* tree-inline.c (remap_gimple_stmt): Add a checked cast to
	gimple_omp_parallel within GIMPLE_OMP_PARALLEL case of switch
	statement, introducing local.