Concretize gimple_cond_set_{lhs|rhs}

Concretize gimple_cond_set_{lhs|rhs}

This corresponds to:
  [PATCH 75/89] Concretize gimple_cond_set_{lhs|rhs}
  https://gcc.gnu.org/ml/gcc-patches/2014-04/msg01222.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/msg00850.html

gcc/
	* gimple.h (gimple_cond_set_lhs): Require a gimple_cond.
	(gimple_cond_set_rhs): Likewise.

	* tree-parloops.c (transform_to_exit_first_loop): Convert locals
	"cond_stmt" and "cond_nit" to gimple_cond, adding checked casts,
	since the existing code assumes that code is GIMPLE_COND.  Convert
	"stmt" to a gimple_assign.
	(create_parallel_loop): Likewise for "cond_stmt".

	* tree-ssa-loop-im.c (rewrite_bittest): Replace check for code
	GIMPLE_COND with a dyn_cast, adding new local "cond_stmt".