[Target] Avoid repeated hash lookups (NFC) (#108677)

Author: kazutakahirata

llvm/lib/Target/ARM/ARMISelLowering.cpp

@@ -8017,7 +8017,6 @@ SDValue ARMTargetLowering::LowerBUILD_VECTOR(SDValue Op, SelectionDAG &DAG,
     if (!isa<ConstantFPSDNode>(V) && !isa<ConstantSDNode>(V))
       isConstant = false;
 
-    ValueCounts.insert(std::make_pair(V, 0));
     unsigned &Count = ValueCounts[V];
 
     // Is this value dominant? (takes up more than half of the lanes)

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -3679,7 +3679,6 @@ static SDValue lowerBuildVectorViaDominantValues(SDValue Op, SelectionDAG &DAG,
     if (V.isUndef())
       continue;
 
-    ValueCounts.insert(std::make_pair(V, 0));
     unsigned &Count = ValueCounts[V];
     if (0 == Count)
       if (auto *CFP = dyn_cast<ConstantFPSDNode>(V))