[FIX] Hoist accesses if AA stated they are invariant

  Before this patch it could happen that we did not hoist a load that
  was a base pointer of another load even though AA already declared the
  first one as invariant (during ScopDetection). If this case arises we
  will now skipt the "can be overwriten" check because in this case the
  over-approximating nature causes us to generate broken code.

llvm-svn: 260862