Split code out of vect_transform_slp_perm_load

Split code out of vect_transform_slp_perm_load

Similarly to the previous vectorizable_slp_permutation patch,
this one splits out the main part of vect_transform_slp_perm_load
so that a later patch can test a permutation without constructing
a node for it.

Also fixes a lingering use of STMT_VINFO_VECTYPE.

gcc/
	* tree-vect-slp.cc (vect_transform_slp_perm_load_1): Split out from...
	(vect_transform_slp_perm_load): ...here.  Use SLP_TREE_VECTYPE instead
	of STMT_VINFO_VECTYPE.