[NFC][ConstantFold] Check getAggregateElement before getSplatValue call

[zsrkmyn]

Summary

Constant::getSplatValue has O(N) time complexity in the worst case,
where N is the # of elements in a vector. So we call
Constant::getAggregateElement first and return earlier if possible to
avoid unnecessary getSplatValue calls.

[MaskRay]

Thanks!

[zsrkmyn]

Would you mind helping me commit it if there's no further comments? Many thanks :-)

llvm/lib/IR/ConstantFold.cpp

if (auto *GEP = dyn_cast<GEPOperator>(CE)) {
           return CE->getOperand(1);
         } else {
           return ConstantExpr::getExtractElement(CE->getOperand(0), CIdx);
         }
       }
     }
   }
 
+  if (Constant *C = Val->getAggregateElement(CIdx))
+    return C;
+
   // Lane < Splat minimum vector width => extractelt Splat(x), Lane -> x
   if (CIdx->getValue().ult(ValVTy->getElementCount().getKnownMinValue())) {
     if (Constant *SplatVal = Val->getSplatValue())
       return SplatVal;
   }
 
-  return Val->getAggregateElement(CIdx);
+  return nullptr;
 }
 
 Constant *llvm::ConstantFoldInsertElementInstruction(Constant *Val,
                                                      Constant *Elt,
                                                      Constant *Idx) {
   if (isa<UndefValue>(Idx))
     return PoisonValue::get(Val->getType());