[ValueTracking] fix miscompile in maxnum case of cannotBeOrderedLessThanZeroImpl (PR46627)
A miscompile with -0.0 is shown in:
This is because maxnum(-0.0, +0.0) does not specify a fixed result:
So we need to tighten the constraints for when it is ok to say the
result of maxnum is positive (including +0.0).
Differential Revision: https://reviews.llvm.org/D83601