[FIX] Handle multiplications in the SCEVAffinator again

  If ScalarEvolution cannot look through some expression but we do, it
  might happen that a multiplication will arrive at the
  SCEVAffinator::visitMulExpr. While we could always try to improve the
  extractConstantFactor function we might still miss something, thus we
  reintroduce the code to generate multiplicative piecewise-affine
  functions as a fall-back.

llvm-svn: 265777