This allows us to recognise more saturation patterns and also simplify some MINMAX codegen that was failing to simplify CMPGE comparisons to a legal CMPGT.
An alternative would be to allow SelectionDAGBuilder to always create MINMAX ops and improve the legalizers vector support - is an all-target solution something we'd want?
Can we do this in reverse too? SMIN/SMAX->UMIN/UMAX?