Fix nasty recompilation bug in MkIface.computeChangedOccs

Fix nasty recompilation bug in MkIface.computeChangedOccs

	MERGE to 6.8 branch

In computeChangedOccs we look up the old version of a Name.
But a WiredIn Name doesn't have an old version, because WiredIn things
don't appear in interface files at all.

Result: ghc-6.9: panic! (the 'impossible' happened)
  (GHC version 6.9 for x86_64-unknown-linux):
	lookupVers1 base:GHC.Prim chr#{v}

This fixes the problem.  The patch should merge easily onto the branch.