tree-parloops.c: use gimple_phi in various places

tree-parloops.c: use gimple_phi in various places

This corresponds to:
  [PATCH 11/89] tree-parloops.c: use gimple_phi in various places
  https://gcc.gnu.org/ml/gcc-patches/2014-04/msg01149.html
from the original 89-patch kit

That earlier patch was approved by Jeff:
> OK when prerequisites have gone in.
in https://gcc.gnu.org/ml/gcc-patches/2014-05/msg00620.html

gcc/
	* tree-parloops.c (struct reduction_info): Strengthen field "new_phi"
	from gimple to gimple_phi.
	(create_phi_for_local_result): Convert local "new_phi" to gimple_phi.
	(loop_has_vector_phi_nodes): Require a gimple_phi rather than a gimple.
	(gather_scalar_reductions): Convert to a gimple_phi_iterator and
	gimple_phi.
	(try_create_reduction_list): Likewise.