[FIX] Carefully rewrite parameters wrt. invariant equivalence classes

  ScalarEvolution doesn't allow the operands of an AddRec to be variant in the
  loop of the AddRec. When we rewrite parameter SCEVs it might seem like the
  new SCEV violates this property and ScalarEvolution will trigger an
  assertion. To avoid this we move the start part out of an AddRec when we
  rewrite it, thus avoid the operands to be possibly variant completely.

llvm-svn: 251945