tree-optimization/117307 - STMT_VINFO_SLP_VECT_ONLY mis-computation

tree-optimization/117307 - STMT_VINFO_SLP_VECT_ONLY mis-computation

STMT_VINFO_SLP_VECT_ONLY isn't properly computed as union of all
group members and when the group is later split due to duplicates
not all sub-groups inherit the flag.

	PR tree-optimization/117307
	* tree-vect-data-refs.cc (vect_analyze_data_ref_accesses):
	Properly compute STMT_VINFO_SLP_VECT_ONLY.  Set it on all
	parts of a split group.

	* gcc.dg/vect/pr117307.c: New testcase.

(cherry picked from commit 19722308a286d9a00eead8ac82b948da8c4ca38b)