Separate more constant factors of parameters

  So far we separated constant factors from multiplications, however,
  only when they are at the outermost level of a parameter SCEV. Now,
  we also separate constant factors from the parameter SCEV if the
  outermost expression is a SCEVAddRecExpr. With the changes to the
  SCEVAffinator we can now improve the extractConstantFactor(...)
  function at will without worrying about any other code part. Thus,
  if needed we can implement a more comprehensive
  extractConstantFactor(...) function that will traverse the SCEV
  instead of looking only at the outermost level.

  Four test cases were affected. One did not change much and the other
  three were simplified.

llvm-svn: 260859