[LAA] Fix the build with older versions of Clang

llvm/lib/Analysis/LoopAccessAnalysis.cpp:916:12: error: no viable conversion from returned value of type 'SmallVector<[...], 2>' to function return type 'SmallVector<[...], (default)
      CalculateSmallVectorDefaultInlinedElements<T>::value aka 3>'
    return Scevs;
           ^~~~~