[AArch64] Build v2i64 Mul cost out of getArithmeticInstrCost and getVectorInstrCost. NFCI

This should not effect the result, unless the getArithmeticInstrCost and
getVectorInstrCost routines learn to produce different costs (with CostKind =
CodeSize for example). The -1 lanes prevent 0 lanes from (incorrectly) being
marked as free.

Author: davemgreen

llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp

@@ -3623,7 +3623,13 @@ InstructionCost AArch64TTIImpl::getArithmeticInstrCost(
     // so the cost can be cheaper (smull or umull).
     if (LT.second != MVT::v2i64 || isWideningInstruction(Ty, Opcode, Args))
       return LT.first;
-    return LT.first * 14;
+    return cast<VectorType>(Ty)->getElementCount().getKnownMinValue() *
+           (getArithmeticInstrCost(Opcode, Ty->getScalarType(), CostKind) +
+            getVectorInstrCost(Instruction::ExtractElement, Ty, CostKind, -1,
+                               nullptr, nullptr) *
+                2 +
+            getVectorInstrCost(Instruction::InsertElement, Ty, CostKind, -1,
+                               nullptr, nullptr));
   case ISD::ADD:
   case ISD::XOR:
   case ISD::OR: