tree-parloops.c: Use gimple_phi in various places

tree-parloops.c: Use gimple_phi in various places

This corresponds to:
  [PATCH 46/89] tree-parloops.c: Use gimple_phi in various places
  https://gcc.gnu.org/ml/gcc-patches/2014-04/msg01168.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/msg00819.html

gcc/
	* tree-parloops.c (reduction_info::keep_res): Strengthen field
	from plain gimple to gimple_phi.
	(transform_to_exit_first_loop): Strengthen locals "phi", "nphi"
	to gimple_phi.  Eliminate early decl of gimple_stmt_iterator gsi
	in favor of more tightly scoped gimple_phi_iterators, and a final
	later decl as a gimple_stmt_iterator.