[SelectionDAGBuilder] Save iterator to avoid second DenseMap lookup. NFC

We were calling find and then using operator[]. Instead keep the
iterator from find and use it to get the value.

Just happened to notice while investigating how we decide what extends
to use between basic blocks.

llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

@@ -9873,10 +9873,10 @@ SelectionDAGBuilder::CopyValueToVirtualRegister(const Value *V, unsigned Reg) {
                    None); // This is not an ABI copy.
   SDValue Chain = DAG.getEntryNode();
 
-  ISD::NodeType ExtendType = (FuncInfo.PreferredExtendType.find(V) ==
-                              FuncInfo.PreferredExtendType.end())
-                                 ? ISD::ANY_EXTEND
-                                 : FuncInfo.PreferredExtendType[V];
+  ISD::NodeType ExtendType = ISD::ANY_EXTEND;
+  auto PreferredExtendIt = FuncInfo.PreferredExtendType.find(V);
+  if (PreferredExtendIt != FuncInfo.PreferredExtendType.end())
+    ExtendType = PreferredExtendIt->second;
   RFV.getCopyToRegs(Op, DAG, getCurSDLoc(), Chain, nullptr, V, ExtendType);
   PendingExports.push_back(Chain);
 }