TLC to vect_check_store_rhs and vect_slp_child_index_for_operand

TLC to vect_check_store_rhs and vect_slp_child_index_for_operand

This prepares us for the SLP of scatters.  We have to tell
vect_slp_child_index_for_operand whether we are dealing with
a scatter/gather stmt so this adds an argument similar to
the one we have for vect_get_operand_map.  This also refactors
vect_check_store_rhs to get the actual rhs and the associated
SLP node instead of leaving that to the caller.

	* tree-vectorizer.h (vect_slp_child_index_for_operand):
	Add gatherscatter_p argument.
	* tree-vect-slp.cc (vect_slp_child_index_for_operand): Likewise.
	Pass it on.
	* tree-vect-stmts.cc (vect_check_store_rhs): Turn the rhs
	argument into an output, also output the SLP node associated
	with it.
	(vectorizable_simd_clone_call): Adjust.
	(vectorizable_store): Likewise.
	(vectorizable_load): Likewise.