Fix GROUP_GAP for single-element interleaving

Fix GROUP_GAP for single-element interleaving

vectorizable_load had a curious "force_peeling" variable, with no
comment explaining why we need it for single-element interleaving
but not for other cases.  I think it's simply because we weren't
initialising the GROUP_GAP correctly for single loads.

Tested on aarch64-linux-gnu and x86_64-linux-gnu.

gcc/
	* tree-vect-data-refs.c (vect_analyze_group_access_1): Set
	GROUP_GAP for single-element interleaving.
	* tree-vect-stmts.c (vectorizable_load): Remove force_peeling
	variable.

From-SVN: r236631