APFloat: Fix maxnum and minnum with sNaN (#112854)
See: https://github.com/llvm/llvm-project/pull/112852
Fixes: https://github.com/llvm/llvm-project/issues/111991
We have reclarify llvm.maxnum and llvm.minnum to follow IEEE-754 2008's
maxNum and minNum with +0.0>-0.0.
So let's make APFloat::maxnum and APFloat::minnum to follow it, too.
