Concretize gimple_cond_make_{false|true}

Concretize gimple_cond_make_{false|true}

This corresponds to:
  [PATCH 71/89] Concretize gimple_cond_make_{false|true}
  https://gcc.gnu.org/ml/gcc-patches/2014-04/msg01208.html
from the original 89-patch kit

That earlier patch was approved by Jeff:
> OK once prerequisites go in.
in https://gcc.gnu.org/ml/gcc-patches/2014-05/msg00863.html

gcc/
	* gimple.h (gimple_cond_make_false): Require a gimple_cond.
	(gimple_cond_make_true): Likewise.

	* tree-cfg.c (fold_cond_expr_cond): Add a checked cast to
	gimple_cond within region guarded by check for GIMPLE_COND.
	* tree-ssa-ccp.c (ccp_fold_stmt): Likewise.

	* tree-loop-distribution.c (generate_loops_for_partition): Replace
	a check for GIMPLE_COND with a dyn_cast<gimple_cond>.
	* tree-ssa-ccp.c (optimize_unreachable): Likewise.
	* tree-ssa-loop-niter.c (number_of_iterations_exit): Likewise.
	* tree-ssa-pre.c (eliminate_dom_walker::before_dom_children):
	Likewise.

	* tree-vrp.c (fold_predicate_in): Add a checked cast to
	gimple_cond.  We must be dealing with a GIMPLE_COND since logic
	at top of the function ensures we only act on GIMPLE_ASSIGN and
	GIMPLE_COND statements, and we're now within a "not a GIMPLE_ASSIGN"
	clause.

	* tree-ssa-loop-ivcanon.c (remove_exits_and_undefined_stmts): Add
	checked cast of elt->stmt to gimple_cond.  The existing code requires
	this to be a GIMPLE_COND, though it's not clear to me how this
	requirement is enforced.
	(remove_redundant_iv_tests): Likewise.
	(try_unroll_loop_completely): Likewise, for the last_stmt of the
	preceding bb along edge_to_cancel.
	* tree-ssa-reassoc.c (maybe_optimize_range_tests): Likewise, for the
	last_stmt of bb.