tree-optimization/117558 - peeling for gaps and VL vectors

The following ensures that peeling a single iteration for gaps is
sufficient by enforcing niter masking (partial vector use) given
we cannot (always) statically decide when the vector size isn't known.
The condition guarding this and thus statically giving a pass in
some cases for VL vectors is questionable, the patch doesn't address
this.

This fixes a set of known failout from enabling
--param vect-force-slp=1 by default.

	PR tree-optimization/117558
	* tree-vectorizer.h (_loop_vec_info::must_use_partial_vectors_p): New.
	(LOOP_VINFO_MUST_USE_PARTIAL_VECTORS_P): Likewise.
	* tree-vect-loop.cc (_loop_vec_info::_loop_vec_info): Initialize
	must_use_partial_vectors_p.
	(vect_determine_partial_vectors_and_peeling): Enforce it.
	(vect_analyze_loop_2): Reset before restarting.
	* tree-vect-stmts.cc (get_group_load_store_type): When peeling
	a single gap iteration cannot be determined safe statically
	enforce the use of partial vectors.