Introduce gimple_goto

Introduce gimple_goto

This corresponds to:
  [PATCH 22/89] Introduce gimple_goto
  https://gcc.gnu.org/ml/gcc-patches/2014-04/msg01174.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_goto): New typedef.
	(const_gimple_goto): New typedef.

	* gimple.h (gimple_statement_goto): New subclass of
	gimple_statement_with_ops, adding the invariant that
	stmt->code == GIMPLE_GOTO.
	(is_a_helper <gimple_statement_goto>::test): New.
	(gimple_build_goto): Return a gimple_goto rather than a
	plain gimple.

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

	* gimple.c (gimple_build_goto): Return a gimple_goto rather than a
	plain gimple.

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