Fix gcc.dg/vect/vect-early-break_39.c FAIL with forced SLP

The testcases shows single-element interleaving of size three
being exempted from permutation lowering via heuristics
(see also PR116973).  But it wasn't supposed to apply to
non-power-of-two sizes so this amends the check to ensure
the sub-group is aligned even when the number of lanes is one.

	* tree-vect-slp.cc (vect_lower_load_permutations): Avoid
	exempting non-power-of-two group sizes from lowering.