Make gimple_phi_arg_def_ptr and gimple_phi_arg_has_location require a gimple_phi

Make gimple_phi_arg_def_ptr and gimple_phi_arg_has_location require a gimple_phi

This corresponds to:
  [PATCH 48/89] Make gimple_phi_arg_def_ptr and gimple_phi_arg_has_location require a gimple_phi
  https://gcc.gnu.org/ml/gcc-patches/2014-04/msg01197.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/msg00828.html

gcc/
	* gimple.h (gimple_phi_arg_def_ptr): Require a gimple_phi rather
	than a plain gimple.
	(gimple_phi_arg_has_location): Likewise.

	* gimple-streamer-in.c (input_phi): Return a gimple_phi rather
	than a plain gimple.
	* gimple-streamer-out.c (output_phi): Require a gimple_phi rather
	than a plain gimple.
	(output_bb): Convert iteration to a gimple_phi_iterator, and local
	"phi" to gimple_phi.

	* omp-low.c (expand_omp_for_static_chunk): Convert iterator "psi"
	to a gimple_phi_iterator; convert locals "phi" and "nphi" to be
	gimple_phi.

	* tree-cfg.c (gimple_duplicate_sese_tail): Likewise for "psi" and
	"phi".
	(move_block_to_fn): Introduce new gimple_phi_iterator "psi", using
	it in place of "gsi" where necessary.  Convert "phi" to be a
	gimple_phi.

	* tree-cfgcleanup.c (remove_forwarder_block): Likewise.

	* tree-vect-loop-manip.c (vect_loop_versioning): Convert "gsi" to
	a gimple_phi_iterator, and "orig_phi" and "new_phi" to be
	gimple_phi.

	* tree.c (find_decls_types_in_node): Introduce new
	gimple_phi_iterator "psi", using it in place of "si" where
	necessary.  Convert "phi" to be a gimple_phi.