vect: Fix check_reduction_path [PR112374]

As mentioned in the PR, the intent of the r14-5076 changes was that
it doesn't count one of the uses on the use_stmt, but what actually
got implemented is that it does this processing on any op_use_stmt,
even if it is not the use_stmt statement, which means that it
can increase count even on debug stmts (-fcompare-debug failures),
or if there would be some other use stmt with 2+ uses it could count
that as a single use.  Though, because it fails whenever cnt != 1
and I believe use_stmt must be one of the uses, it would probably
fail in the latter case anyway.

The following patch fixes that by doing this extra processing only when
op_use_stmt is use_stmt, and using the normal processing otherwise
(so ignore debug stmts, and increase on any uses on the stmt).

2023-11-17  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/112374
	* tree-vect-loop.cc (check_reduction_path): Perform the cond_fn_p
	special case only if op_use_stmt == use_stmt, use as_a rather than
	dyn_cast in that case.

	* gcc.dg/pr112374-1.c: New test.
	* gcc.dg/pr112374-2.c: New test.
	* g++.dg/opt/pr112374.C: New test.