EscapeAnalysis: Add PointerKind and interior/reference flags

Categorize three kinds of pointers:

NoPointer (don't create a node)

ReferenceOnly (safe to make normal assumptions)

AnyPointer (may have addresses, rawpointers, or any mix of thoses with references)

Flag ConnectionGraph nodes as
- hasReferenceOnly
- isInterior

An interior node always has an additional content node.

All sorts of arbitrary node merging is supported. Nodes with totally
different properties can be safely merged. Interior nodes can safely
be merged with their field content (which does happen surprisingly
often).

Alias analysis will use these flags to safely make assumptions about
properties of the connection graph.

Author: atrick