[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;
           ^~~~~

llvm/lib/Analysis/LoopAccessAnalysis.cpp

@@ -908,7 +908,7 @@ findForkedPointer(PredicatedScalarEvolution &PSE,
                   const Loop *L) {
   ScalarEvolution *SE = PSE.getSE();
   assert(SE->isSCEVable(Ptr->getType()) && "Value is not SCEVable!");
-  SmallVector<std::pair<const SCEV *, bool>, 2> Scevs;
+  SmallVector<std::pair<const SCEV *, bool>> Scevs;
   findForkedSCEVs(SE, L, Ptr, Scevs, MaxForkedSCEVDepth);
 
   // For now, we will only accept a forked pointer with two possible SCEVs.