[SDAG] Check fminnum/fmaxnum for non-zero operand.

Currently, in TargetLowering, if the target does not support fminnum, we lower
to fminimum if neither operand could be a NaN. But this isn't quite correct
because fminnum and fminimum treat +/-0 differently; so, we need to prove that
one of the operands isn't a zero, or we don't have signed zeros.

Differential Revision: https://reviews.llvm.org/D143256