Concretize gimple_cond_{true|false}_label

Concretize gimple_cond_{true|false}_label

This corresponds to:
  [PATCH 73/89] Concretize gimple_cond_{true|false}_label
  https://gcc.gnu.org/ml/gcc-patches/2014-04/msg01169.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/msg00845.html

gcc/
	* gimple.h (gimple_cond_true_label): Require a const_gimple_cond
	rather than just a const_gimple.
	(gimple_cond_false_label): Likewise.

	* omp-low.c (diagnose_sb_2): Add checked cast to gimple_cond within
	case GIMPLE_COND.
	* tree-eh.c (maybe_record_in_goto_queue): Likewise.

	* tree-cfg.c (verify_gimple_stmt): Add a checked cast to gimple_cond
	within "case GIMPLE_COND", splitting it out into...
	(verify_gimple_label): New.