[Statepoint] Sink logic about actual callee into GCStatepointInst

Sinking logic around actual callee from Statepoint to GCStatepointInst.  While doing so, adjust naming to be consistent about refering to "actual" callee and follow precedent on naming from CallBase otherwise.

Use the result to simplify one consumer.  This is mostly just to ensure the new code is exercised, but is also a helpful cleanup on it's own.