Introduce gimple_omp_for

Introduce gimple_omp_for

This corresponds to:
  [PATCH 38/89] Introduce gimple_omp_for
  https://gcc.gnu.org/ml/gcc-patches/2014-04/msg01225.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/msg00825.html

gcc/
	* coretypes.h (gimple_omp_for): New.
	(const_gimple_omp_for): New.

	* gimple.h (gimple_build_omp_for): Return a gimple_omp_for rather
	than a plain gimple.
	(gimple_omp_for_set_kind): Require a gimple_omp_for rather than a
	plain gimple.
	(gimple_omp_for_set_combined_p): Likewise.
	(gimple_omp_for_set_combined_into_p): Likewise.

	* gimple-pretty-print.c (dump_gimple_omp_for): Require a
	gimple_omp_for rather than a plain gimple.
	(pp_gimple_stmt_1): Add a checked cast to gimple_omp_for in
	GIMPLE_OMP_FOR case of switch statement.

	* gimple.c (gimple_build_omp_for): Return a gimple_omp_for rather
	than a plain gimple.
	(gimple_copy): Add a checked cast to gimple_omp_for and a new local.

	* gimplify.c (gimplify_omp_for): Strengthen local "gfor" from
	gimple to gimple_omp_for.

	* omp-low.c (omp_for_data::for_stmt): Strengthen field from gimple
	to gimple_omp_for.
	(extract_omp_for_data): Require a gimple_omp_for rather than a
	plain gimple.
	(workshare_safe_to_combine_p): Add a checked cast to
	gimple_omp_for.
	(get_ws_args_for): Convert check of code against GIMPLE_OMP_FOR
	with a dyn_cast<gimple_omp_for> and a new local.
	(scan_omp_parallel): Add a checked cast to gimple_omp_for and a
	new local.
	(scan_omp_for): Require a gimple_omp_for rather than a plain
	gimple.
	(scan_omp_1_stmt): Add a checked cast to gimple_omp_for in
	GIMPLE_OMP_FOR case of switch statement.
	(expand_omp_for): Add a checked cast to gimple_omp_for.
	(lower_omp_for): Strengthen local "stmt" from gimple to
	gimple_omp_for.

	* tree-nested.c (walk_gimple_omp_for): Require a gimple_omp_for
	rather than a plain gimple.
	(convert_nonlocal_reference_stmt): Add a checked cast to
	gimple_omp_for in GIMPLE_OMP_FOR case of switch statement.
	(convert_local_reference_stmt): Likewise.

	* tree-parloops.c (create_parallel_loop): Strengthen local
	"for_stmt" from gimple to gimple_omp_for.