Fix fallout of peeling for gap improvements

Fix fallout of peeling for gap improvements

The following hopefully addresses an observed bootstrap issue on aarch64
where maybe-uninit diagnostics occur.  It also fixes bogus napkin math
from myself when I was confusing rounded up size of a single access
with rounded up size of the group accessed in a single scalar iteration.
So the following puts in a correctness check, leaving a set of peeling
for gaps as insufficient.  This could be rectified by splitting the
last load into multiple ones but I'm leaving this for a followup, better
quickly fix the reported wrong-code.

	* tree-vect-stmts.cc (get_group_load_store_type): Do not
	re-use poly-int remain but re-compute with non-poly values.
	Verify the shortened load is good enough to be covered with
	a single scalar gap iteration before accepting it.

	* gcc.dg/vect/pr115385.c: Enable AVX2 if available.