Enhance gather fallback for PR65518 with SLP

Enhance gather fallback for PR65518 with SLP

With SLP forced we fail to use gather for PR65518 on RISC-V as expected
because we're failing due to not effective peeling for gaps.  The
following appropriately moves the memory_access_type adjustment before
doing all the overrun checking since using VMAT_ELEMENTWISE means
there's no overrun.

	* tree-vect-stmts.cc (get_group_load_store_type): Move
	VMAT_ELEMENTWISE fallback for single-element interleaving
	of too large groups before overrun checking.

	* gcc.dg/vect/pr65518.c: Adjust.