[SCEV] Remove extra APInt copies from getRangeForAffineARHelper.

This changes one parameter to be a const APInt& since we only read from it. Use std::move on local APInts once they are no longer needed so we can reuse their allocations. Lastly, use operator+=(uint64_t) instead of adding 1 to an APInt twice creating a new APInt each time.

llvm-svn: 302335