tree-optimization/111915 - mixing grouped and non-grouped accesses

The change to allow SLP of non-grouped accesses failed to check
for the case of mixing with grouped accesses.

	PR tree-optimization/111915
	* tree-vect-slp.cc (vect_build_slp_tree_1): Check all
	accesses are either grouped or not.

	* gcc.dg/vect/pr111915.c: New testcase.