Introduce gimple_eh_else

Introduce gimple_eh_else

This corresponds to:
  [PATCH 28/89] Introduce gimple_eh_else
  https://gcc.gnu.org/ml/gcc-patches/2014-04/msg01156.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_eh_else): New typedef.
	(const_gimple_eh_else): New typedef.

	* gimple.h (gimple_build_eh_else): Return a gimple_eh_else rather
	than a plain gimple.
	(gimple_eh_else_n_body_ptr): Require a gimple_eh_else rather than
	a plain gimple.
	(gimple_eh_else_n_body): Likewise.
	(gimple_eh_else_e_body_ptr): Likewise.
	(gimple_eh_else_e_body): Likewise.
	(gimple_eh_else_set_n_body): Likewise.
	(gimple_eh_else_set_e_body): Likewise.

	* gimple-low.c (lower_stmt): Add checked cast to gimple_eh_else
	within GIMPLE_EH_ELSE case of switch statement, introducing a new
	local.
	(gimple_stmt_may_fallthru): Likewise.

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

	* gimple-walk.c (walk_gimple_stmt): Add checked cast to
	gimple_eh_else within GIMPLE_EH_ELSE case of switch statement,
	introducing a new local.

	* gimple.c (gimple_build_eh_else): Return a gimple_eh_else
	rather than a plain gimple.
	(gimple_copy): Add checked casts to gimple_eh_else within
	GIMPLE_EH_ELSE case of switch statement, introducing new locals.

	* tree-cfg.c (verify_gimple_in_seq_2): Add checked cast to
	gimple_eh_else within GIMPLE_EH_ELSE case of switch statement,
	introducing a new local.

	* tree-eh.c (collect_finally_tree): Likewise.
	(replace_goto_queue_1): Likewise.
	(get_eh_else): Return a gimple_eh_else rather than a plain gimple.
	(honor_protect_cleanup_actions): Convert local "eh_else" from
	gimple to gimple_eh_else.
	(lower_try_finally_nofallthru): Likewise.
	(lower_try_finally_onedest): Introduce locals "eh_else" and
	"label_stmt", using them in favor of "x" for the gimple_eh_else
	and the gimple_label.
	(lower_try_finally_copy): Convert local "eh_else" from gimple to
	gimple_eh_else.
	(lower_try_finally_switch): Likewise.
	(decide_copy_try_finally): Likewise.
	(refactor_eh_r): Add checked cast to gimple_eh_else within
	GIMPLE_EH_ELSE case of switch statement, introducing a new local.