SLP: Have vectorizable_slp_permutation set type on invariants

This modifies vectorizable_slp_permutation to update the type of the children
of a perm node before trying to permute them.  This allows us to be able to
permute invariant nodes.

This will be covered by test from the SLP pattern matcher.

gcc/ChangeLog:

	* tree-vect-slp.c (vectorizable_slp_permutation): Update types on nodes
	when needed.