[DAG] matchBinOpReduction - match subvector reduction patterns beyond a matched shuff...

[DAG] matchBinOpReduction - match subvector reduction patterns beyond a matched shufflevector reduction

Currently matchBinOpReduction only handles shufflevector reduction patterns, but in many cases these only occur in the final stages of a reduction, once we're down to legal vector widths.

Before this its likely that we are performing reductions using subvector extractions to repeatedly split the source vector in half and perform the binop on the halves.

Assuming we've found a non-partial reduction, this patch continues looking for subvector reductions as far as it can beyond the last shufflevector.

Fixes PR37890