Concretize three gimple_try_set_ accessors

Concretize three gimple_try_set_ accessors

This corresponds to:
  [PATCH 65/89] Concretize three gimple_try_set_ accessors
  https://gcc.gnu.org/ml/gcc-patches/2014-04/msg01167.html
from the original 89-patch kit

That earlier patch was approved by Jeff:
> OK once prerequisites have gone in.
in https://gcc.gnu.org/ml/gcc-patches/2014-05/msg00831.html

gcc/
	* gimple.c (gimple_copy): Add checked casts to gimple_try.

	* gimple.h (gimple_try_set_kind): Require a gimple_try.
	(gimple_try_set_eval): Likewise.
	(gimple_try_set_cleanup): Likewise.

	* tree-eh.c (optimize_double_finally): Require a pair of gimple_try
	statements.
	(refactor_eh_r): Convert code comparisons to dynamic casts.