Unify last two vect_transform_slp_perm_load calls

Unify last two vect_transform_slp_perm_load calls

The following unifies the vect_transform_slp_perm_load call done
in vectorizable_load with that eventually done in get_load_store_type.
On the way it fixes the conditions on which we can allow
VMAT_ELEMENTWISE or VMAT_GATHER_SCATTER when there's a SLP permutation
(and we arrange to not code generate that).  In particular that only
works for single-lane SLP of non-grouped loads or groups of size one.
VMAT_ELEMENTWISE does not (yet?) materialize a permutation upon
vector build but still relies on vect_transform_slp_perm_load.

	* tree-vect-stmts.cc (get_load_store_type): Get in a flag
	whether a SLP_TREE_LOAD_PERMUTATION on the node can be
	code generated and use it.  Fix the condition on using
	strided gather/scatter to avoid dropping a meaningful
	permutation.
	(vectorizable_store): Adjust.
	(vectorizable_load): Analyze the permutation early and
	pass the result down to get_load_store_type.  Fix the
	condition on when we are allowed to elide a load permutation.