Introduce gimple_return

Introduce gimple_return

This corresponds to:
  [PATCH 21/89] Introduce gimple_return
  https://gcc.gnu.org/ml/gcc-patches/2014-04/msg01180.html
from the original 89-patch kit

That earlier patch was approved by Jeff:
> OK after fixing up the naming/const stuff as discussed for prior
> patches.
> That applies to 22-30. Make sure to take care of
> the pretty printers per Trevor's comments as well. He indicated those
> were missing in a couple of those patches.
in https://gcc.gnu.org/ml/gcc-patches/2014-05/msg00628.html

gcc/
	* coretypes.h (gimple_return): New typedef.
	(const_gimple_return): New typedef.

	* gimple.h (gimple_statement_return): New subclass of
	gimple_statement_with_memory_ops, adding the invariant that
	stmt->code == GIMPLE_RETURN.
	(is_a_helper <gimple_statement_return>::test): New.
	(gimple_build_return): Return a gimple_return rather
	than a plain gimple.

	* gimple.c (gimple_build_return): Return a gimple_return rather
	than a plain gimple.

	* cgraphunit.c (expand_thunk): Convert local from a gimple to
	a gimple_return.

	* gimple-low.c (struct return_statements_t): Convert field "stmt"
	from a gimple to a gimple_return.
	(lower_gimple_return): Convert local from a gimple to a
	gimple_return.

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

	* gimplify.c (gimplify_return_expr): Convert locals from
	gimple to gimple_return.

	* ipa-split.c (split_function): Likewise.

	* tree-cfg.c (verify_gimple_assign): Require a gimple_return
	rather than a plain gimple.
	(verify_gimple_stmt): Add checked cast to gimple_return within
	case GIMPLE_RETURN of switch statement.

	* tree-tailcall.c (adjust_return_value): Convert local from
	gimple to gimple_return.