Do not cost the permute node that are part of SLP load-lanes

Do not cost the permute node that are part of SLP load-lanes

There are some SVE testsuite fails when forcing SLP because costing
prevents VLA vectors from being used as we add permute cost for
the VEC_PERM nodes that are part of a SLP load-lanes node.  The
permutes only exist for representational reasons and pessimize SLP
vs non-SLP so the following makes sure to cost them as zero.

	* tree-vect-slp.cc (vectorizable_slp_permutation_1): Return
	zero for the permute nodes part of load-lanes.