[DAG] Fix and expand fmin/fmax reassociation fold.

This call to reassociateReduction is used by both fminnum/fmaxnum and
fminimum/fmaximum. In adding support for fminimum/fmaximum we appear to be
fixing the use of an incorrect reduction type, which should have only applied
to minnum/maxnum.

I also believe that it doesn't need nsz and reassoc to perform the
reassociation. For float min/max it should always be valid.

Differential Revision: https://reviews.llvm.org/D153247