Introduce a GCStatepointInst type analogous to IntrinsicInst subclasses

Back when we had CallSite, we implemented the current Statepoint/ImmutableStatepoint structure in analogous manner.  Now that CallSite has been removed, the structure used for statepoints looks decidely out of place.  gc.statepoint is one of the small handful of intrinsics which are invokable.  Because of this, it can't subclass IntrinsicInst as is idiomatic.

This change simply introduces the GCStatepointInst class, restructures the existing Statepoint/ImmutableStatepoint types to wrap it.  I will be landing a series of changes to sink functionality into GCStatepointInst and updating callers to be more idiomatic.