[Local] collectBitParts - bail out if we find more than one root input value.

All the uses that we have for collectBitParts revolve around us matching down to an operation with a single root value - I don't think we're intending to change that (and a lot of collectBitParts assumes it).

The binops cases (OR/FSHL/FSHR) already check if the providers are the same, but that would still mean we waste time collecting through unaryops before getting to them.