[DAG] Add SelectionDAG::ComputeMinSignedBits helper

As suggested on D113371, this adds a wrapper to SelectionDAG::ComputeNumSignBits, similar to the llvm::ComputeMinSignedBits wrapper.

I've included some usage, its not exhaustive, just the more obvious cases where the intention is obvious.

Differential Revision: https://reviews.llvm.org/D113396