Concretize various expressions from gimple to gimple_cond

Concretize various expressions from gimple to gimple_cond

This corresponds to:
  [PATCH 77/89] Concretize various expressions from gimple to gimple_cond
  https://gcc.gnu.org/ml/gcc-patches/2014-04/msg01196.html
from the original 89-patch kit

That earlier patch was approved by Jeff:
> OK once prereqs go in.
in https://gcc.gnu.org/ml/gcc-patches/2014-05/msg00858.html

gcc/
	* ipa-split.c (check_forbidden_calls): Replace check against
	GIMPLE_COND with a dyn_cast<gimple_cond>, introducing a
	gimple_cond local.
	* predict.c (predict_extra_loop_exits): Likewise.
	* tree-vrp.c (fold_predicate_in): Likewise.
	(simplify_stmt_for_jump_threading): Likewise.

	* predict.c (is_comparison_with_loop_invariant_p): Require a
	gimple_cond.
	(predict_iv_comparison): Add checked cast to gimple_cond once we
	know the code is GIMPLE_COND.
	(predict_loops): Change type of "stmt" to gimple_cond,
	adding checked casts to its assignments (which are both guarded by
	checks against GIMPLE_COND).

	* tree-vrp.c (find_conditional_asserts): Require a gimple_cond.
	(vrp_evaluate_conditional): Likewise.
	(find_assert_locations_1): Add checked cast to gimple_cond.
	(vrp_visit_stmt): Likewise.