[DAG] Improve FMINNUM/FMAXNUM/FMINIMUM/FMAXIMUM constant folding

Merge the node combines into a common DAGCombiner::visitFMinMax (like we do for IMINMAX).

Move the constant folding into SelectionDAG::foldConstantFPMath.

This allows us to fold the vecreduce-propagate-sd-flags.ll test as it reduces constants - so I've refactored it to take variables instead.

Differential Revision: https://reviews.llvm.org/D115952