[DAG] Enable ISD::SRL SimplifyMultipleUseDemandedBits handling inside SimplifyDemandedBits

This patch allows SimplifyDemandedBits to call SimplifyMultipleUseDemandedBits in cases where the ISD::SRL source operand has other uses, enabling us to peek through the shifted value if we don't demand all the bits/elts.

This is another step towards removing SelectionDAG::GetDemandedBits and just using TargetLowering::SimplifyMultipleUseDemandedBits.

There a few cases where we end up with extra register moves which I think we can accept in exchange for the increased ILP.

Differential Revision: https://reviews.llvm.org/D77804