[SelectionDAG] ComputeNumSignBits - use Valid Min/Max shift amount helpers directly. NFCI.

We are calling getValidShiftAmountConstant first followed by getValidMinimumShiftAmountConstant/getValidMaximumShiftAmountConstant if that failed. But both are used in the same way in ComputeNumSignBits and the Min/Max variants call getValidShiftAmountConstant internally anyhow.