[SelectionDAG] Avoid repeated hash lookups (NFC) (#123697)

Author: kazutakahirata

llvm/lib/CodeGen/SelectionDAG/StatepointLowering.cpp

@@ -871,10 +871,11 @@ SDValue SelectionDAGBuilder::LowerAsSTATEPOINT(
   for (const auto *Relocate : SI.GCRelocates) {
     Value *Derived = Relocate->getDerivedPtr();
     SDValue SD = getValue(Derived);
-    if (!LowerAsVReg.count(SD))
+    auto It = LowerAsVReg.find(SD);
+    if (It == LowerAsVReg.end())
       continue;
 
-    SDValue Relocated = SDValue(StatepointMCNode, LowerAsVReg[SD]);
+    SDValue Relocated = SDValue(StatepointMCNode, It->second);
 
     // Handle local relocate. Note that different relocates might
     // map to the same SDValue.