Fix for Mantis #24651. Allow pointer comparison for internal expressions (in this case check for valid VMT).

nadd.pas, taddnode:
  + add new constructor "create_internal" which adds "nf_internal" to the node's "flags"
  * pass_typecheck_internal: allow pointer comparisons other than "=" and "<>" for nodes which have "nf_internal" set
psub.pas, generate_bodyentry_block:
  * create the addnode using "create_internal" instead of "create" to allow the pointer comparison

+ added test 

git-svn-id: http://svn.freepascal.org/svn/fpc/trunk@25069 3ad0048d-3df7-0310-abae-a5850022a9f2