Make gimple_cond_set_{true|false}_label require gimple_cond.

Make gimple_cond_set_{true|false}_label require gimple_cond.

This corresponds to:
  [PATCH 69/89] Make gimple_cond_set_{true|false}_label require gimple_cond.
  https://gcc.gnu.org/ml/gcc-patches/2014-04/msg01198.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/msg00835.html

gcc/
	* gimple.h (gimple_cond_set_true_label): Require a gimple_cond.
	(gimple_cond_set_false_label): Likewise.

	* tree-cfg.c (make_cond_expr_edges): Convert "entry" from gimple to
	a gimple_cond.
	(cleanup_dead_labels): Introduce a checked cast to a gimple_cond within
	the GIMPLE_COND case.