Introduce gimple_try

Introduce gimple_try

This corresponds to:
  [PATCH 32/89] Introduce gimple_try
  https://gcc.gnu.org/ml/gcc-patches/2014-04/msg01214.html
from the original 89-patch kit

That earlier patch was approved by Jeff:
> OK once const and associated renaming stuff is fixed.
in https://gcc.gnu.org/ml/gcc-patches/2014-05/msg00636.html

gcc/
	* coretypes.h (gimple_try): New typedef.
	(const_gimple_try): New typedef.

	* gimple-low.c (gimple_try_catch_may_fallthru): Require a
	gimple_try rather than a plain gimple.
	(gimple_stmt_may_fallthru): Add checked cast to gimple_try.

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

	* tree-eh.c (finally_tree_node::parent): Strengthen field from
	gimple to gimple_try.
	(record_in_finally_tree): Require a gimple_try rather than a plain
	gimple.
	(collect_finally_tree): Likewise.
	(collect_finally_tree_1): Likewise.
	(struct leh_tf_state::try_finally_expr): Strengthen field from
	gimple to gimple_try.
	(struct leh_tf_state::top_p): Likewise.
	(lower_eh_must_not_throw): Require a gimple_try rather than a
	plain gimple.
	(frob_into_branch_around): Likewise.
	(lower_try_finally_dup_block): Strengthen local from gimple to
	gimple_try.
	(honor_protect_cleanup_actions): Split out uses of "x" into new
	locals "eh_mnt" and "try_stmt" with stronger types.
	(lower_try_finally): Require a gimple_try rather than a plain
	gimple.
	(lower_catch): Likewise.
	(lower_eh_filter): Likewise.
	(lower_eh_must_not_throw): Likewise.
	(lower_cleanup): Likewise.
	(lower_eh_constructs_2): Add checked cast to gimple_try within
	GIMPLE_TRY case of switch statement, introducing new local
	"try_stmt", using it for type-safety.