Index=Var Or Const to Index=Var + Const
  when Var = Var' << N and log2(Const) <= N
  or when Var = (2^M) * (2^N) and log2(Const) <= (M+N)
BUG=none
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/2085383002 .