[Statepoint] Consolidate relocation type tracking [NFC]

Change the way we track how a particular pointer was relocated at a statepoint in selection dag.  Previously, we used an optional<location> for the spill lowering, and a block local Register for the newly introduced vreg lowering.  Combine all three lowerings (norelocate, spill, and vreg) into a single helper class, and keep a single copy of the information.

This is submitted separately as it really does make the code more readible on it's own, but the indirect motivation is to move vreg tracking from StatepointLowering to FunctionLoweringInfo.  This is the last piece needed to support cross block relocations with vregs; that will follow in a separate (non-NFC) patch.