Concretize gimple_cond_set_code

Concretize gimple_cond_set_code

This corresponds to:
  [PATCH 74/89] Concretize gimple_cond_set_code
  https://gcc.gnu.org/ml/gcc-patches/2014-04/msg01209.html
from the original 89-patch kit

That earlier patch was approved by Jeff:
> Fine once prerequisites go in.
in https://gcc.gnu.org/ml/gcc-patches/2014-05/msg00849.html

gcc/
	* gimple.h (gimple_cond_set_code): Require a gimple_cond.

	* tree-complex.c (expand_complex_comparison): Add a checked cast to
	gimple_cond within "case GIMPLE_COND".

	* tree-ssa-loop-ivcanon.c (create_canonical_iv): Convert local "cond"
	to a gimple_cond, adding a checked cast.  The existing code requires
	that the last statement before the exit edge have code GIMPLE_COND,
	though it's not clear to me where this is verified.

	* tree-ssa-loop-ivopts.c (rewrite_use_compare): Add a checked cast
	to gimple_cond on "use->stmt".

	* tree-ssa-loop-manip.c (tree_transform_and_unroll_loop): Convert
	local "exit_if" to gimple_cond, adding a checked cast.  It's not
	clear to me exactly where the GIMPLE_COND-ness of this is
	established, but the existing code requires it.
	(canonicalize_loop_ivs): Similarly for "stmt".

	* tree-ssa-propagate.c (propagate_tree_value_into_stmt): Replace
	a check against GIMPLE_COND with a dyn_cast<gimple_cond>.