[SelectionDAG] Expose the "getValidShiftAmount" helpers available. NFCI.

These are going to be useful in TargetLowering::SimplifyDemandedBits, so expose these helpers outside of SelectionDAG.cpp

Also add an getValidShiftAmountConstant early-out to getValidMinimumShiftAmountConstant/getValidMaximumShiftAmountConstant so we can use them for scalar cases as well.