Split code out of vectorizable_slp_permutation

Split code out of vectorizable_slp_permutation

A later patch needs to test whether the target supports a
lane_permutation_t without having to construct a full SLP
node to test that.  This patch splits out most of the work
of vectorizable_slp_permutation into a subroutine, so that
properties of the permutation can be passed explicitly without
disturbing the main interface.

The new subroutine still uses an slp_tree argument to get things
like the number of lanes and the vector type.  That's a bit clunky,
but it seemed like the least worst option.

gcc/
	* tree-vect-slp.cc (vectorizable_slp_permutation_1): Split out from...
	(vectorizable_slp_permutation): ...here.