Why do we separate them at all? Is it because we can use extra inhabitants? In that case, won't using negative values screw this up when there are entire spare bits?

cc @jckarter

It's better from a layout standpoint to consider all the payload cases first, figure out what extra inhabitants/spare bits you have available, then work out the tagging and non-payload representations afterward. The case numbering is arbitrary and the value witness can munge it into whatever's necessary to interact with the extra inhabitants interface. In fact, for the single-payload case, it's better to make the payload case negative since the extra inhabitant witnesses already use '-1' to mean 'not an extra inhabitant' and start numbering extra inhabitants at 0.

rdar://24057774