[SelectionDAG] Support sign tracking through `{S|U}INT_TO_FP`

Just a minimal amount of easily provable tracking.

Proofs: https://alive2.llvm.org/ce/z/RQYbdw

Closes #82808

Alive2 to has an issue with `(sitofp i1)`, but it can
be verified by hand: https://godbolt.org/z/qKr7hT7s9