Introduce gimple_label and use it in a few places

Introduce gimple_label and use it in a few places

This corresponds to:
  [PATCH 06/89] Introduce gimple_label and use it in a few places
  https://gcc.gnu.org/ml/gcc-patches/2014-04/msg01152.html
from the original 89-patch kit

That earlier patch was approved by Jeff:
> Same as prior patches for gimple_cond and gimple_assign.
in https://gcc.gnu.org/ml/gcc-patches/2014-05/msg00597.html

gcc/
	* coretypes.h (gimple_label): New typedef.
	(const_gimple_label): New typedef.

	* gimple.h (struct gimple_statement_label): New subclass of
	gimple_statement_with_ops, adding the invariant that
	stmt->code == GIMPLE_LABEL.
	(is_a_helper <gimple_statement_label>::test): New.

	* gdbhooks.py (build_pretty_printer): Add gimple_label and its
	variants, reusing the gimple printer.

	* gimple-pretty-print.c (dump_gimple_label): Require a gimple_label
	rather than just a gimple.
	* tree-cfg.c (verify_gimple_label): Likewise.

	* gimple.c (gimple_build_label): Return a gimple_label rather than
	just a gimple.
	* gimple.h (gimple_build_label): Likewise.

	* gimplify.c (gimplify_case_label_expr): Update local to be a
	gimple_label.
	* tree-switch-conversion.c (gen_inbound_check): Likewise.

	* gimple-pretty-print.c (pp_gimple_stmt_1): Add checked cast to
	gimple_label in regions where a stmt is known to have code
	GIMPLE_LABEL.
	* tree-cfg.c (verify_gimple_stmt): Likewise.