tree-optimization/107451 - SLP load vectorization issue

tree-optimization/107451 - SLP load vectorization issue

When vectorizing SLP loads with permutations we can access excess
elements when the load vector type is bigger than the group size
and the vectorization factor covers less groups than necessary
to fill it.  Since we know the code will only access up to
group_size * VF elements in the unpermuted vector we can simply
fill the rest of the vector with whatever we want.  For simplicity
this patch chooses to repeat the last group.

	PR tree-optimization/107451
	* tree-vect-stmts.cc (vectorizable_load): Avoid loading
	SLP group members from group numbers in excess of the
	vectorization factor.

	* gcc.dg/torture/pr107451.c: New testcase.

(cherry picked from commit 7b2cf5041460859ca4f58e5da1308b7ef9129d8b)