[ValueTracking] Fix CannotBeOrderedLessThanZero() for fdiv (PR58046)

When checking the RHS of fdiv, we should set the SignBitOnly flag,
because a negative zero can become -Inf, which is ordered less
than zero.

Fixes https://github.com/llvm/llvm-project/issues/58046.

Differential Revision: https://reviews.llvm.org/D134876