tree-optimization/115252 - enhance peeling for gaps avoidance

tree-optimization/115252 - enhance peeling for gaps avoidance

Code generation for contiguous load vectorization can already deal
with generalized avoidance of loading from a gap.  The following
extends detection of peeling for gaps requirement with that,
gets rid of the old special casing of a half load and makes sure
when we do access the gap we have peeling for gaps enabled.

	PR tree-optimization/115252
	* tree-vect-stmts.cc (get_group_load_store_type): Enhance
	detecting the number of cases where we can avoid accessing a gap
	during code generation.
	(vectorizable_load): Remove old half-vector peeling for gap
	avoidance which is now redundant.  Add gap-aligned case where
	it's OK to access the gap.  Add assert that we have peeling for
	gaps enabled when we access a gap.

	* gcc.dg/vect/slp-gap-1.c: New testcase.