Make gimple_phi_arg_location_from_edge require a gimple_phi

Make gimple_phi_arg_location_from_edge require a gimple_phi

This corresponds to:
  [PATCH 66/89] Make gimple_phi_arg_location_from_edge require a gimple_phi
  https://gcc.gnu.org/ml/gcc-patches/2014-04/msg01199.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/msg00833.html

gcc/
	* gimple.h (gimple_phi_arg_location_from_edge): Require a
	gimple_phi.

	* tree-parloops.c (create_parallel_loop): Split up local variable
	"stmt", introducing other locals for the various statements created
	by this function.  Reuse "stmt" within the phi-handling code, and
	change to type gimple_phi, since this is the only remaining
	"non-phi" user of gimple_phi_arg_location_from_edge.