vect: Add a vect_phi_initial_value helper function

vect: Add a vect_phi_initial_value helper function

This patch adds a helper function called vect_phi_initial_value
for returning the incoming value of a given loop phi.  The main
reason for adding it is to ensure that the right preheader edge
is used when vectorising nested loops.  (PHI_ARG_DEF_FROM_EDGE
itself doesn't assert that the given edge is for the right block,
although I guess that would be good to add separately.)

gcc/
	* tree-vectorizer.h: Include tree-ssa-operands.h.
	(vect_phi_initial_value): New function.
	* tree-vect-loop.c (neutral_op_for_slp_reduction): Use it.
	(get_initial_defs_for_reduction, info_for_reduction): Likewise.
	(vect_create_epilog_for_reduction, vectorizable_reduction): Likewise.
	(vect_transform_cycle_phi, vectorizable_induction): Likewise.