tree-optimization/122364 - reduction chain with conversion

tree-optimization/122364 - reduction chain with conversion

The following handles detecting of a reduction chain wrapped in a
conversion.  This does not yet try to combine operands with different
signedness, but we should now handle signed integer accumulation
to both a signed and unsigned accumulator fine.

	PR tree-optimization/122364
	* tree-vect-slp.cc (vect_analyze_slp_reduc_chain): Re-try
	linearization on a conversion source.

	* gcc.dg/vect/vect-reduc-chain-5.c: New testcase.