tree-optimization/116791 - Elementwise SLP vectorization

The following restricts the elementwise SLP vectorization to the
single-lane case which is the reason I enabled it to avoid regressions
with non-SLP.  The PR shows that multi-line SLP loads with elementwise
accesses require work, I'll open a new bug to track this for the
future.

	PR tree-optimization/116791
	* tree-vect-stmts.cc (get_group_load_store_type): Only
	fall back to elementwise access for single-lane SLP, restore
	hard failure mode for other cases.

	* gcc.dg/vect/pr116791.c: New testcase.