tree-optimization/120687 - reduction chain with UB on signed overflow

tree-optimization/120687 - reduction chain with UB on signed overflow

The following adds the ability to discover a reduction chain on a
series of statements that invoke undefined behavior on integer overflow.
This inhibits the reassoc pass from associating stmts in the way
naturally leading to a reduction chain.  The common mistake on the
source side is to rely on the += operator to sum multiple inputs.

After the refactoring of how we handle reduction chains we can
easily use vect_slp_linearize_chain to do this our selves and
rely on the vectorizer punning operations to unsigned given reduction
vectorization always associates.

	PR tree-optimization/120687
	* tree-vect-slp.cc (vect_analyze_slp_reduc_chain): When
	there's no natural reduction chain see if vect_slp_linearize_chain
	can recover one and built the SLP instance manually in that
	case.
	(vect_schedule_slp): Deal with NULL lanes when looking for
	stores to remove.
	* tree-vect-loop.cc (vect_transform_cycle_phi): Dump when we
	are successfully transforming a reduction chain.

	* gcc.dg/vect/vect-reduc-chain-4.c: New testcase.