tree-optimization/117605 - SLP with large negative single-element interleaving

We fail to demote this to VMAT_ELEMENTWISE and thus run into the three
vector permutation limit (and would not consider to use strided loads
or gathers).

This resolves another bunch of SVE regressions with --param
vect-force-slp=1

	PR tree-optimization/117605
	* tree-vect-stmts.cc (get_group_load_store_type): Also
	apply group size limit for single-element interleaving
	to VMAT_CONTIGUOUS_REVERSE.