[MIRParser] Avoid repeated map lookups (NFC) (#123561)

Author: kazutakahirata

llvm/lib/CodeGen/MIRParser/MIParser.cpp

@@ -1316,9 +1316,10 @@ bool MIParser::parseMachineMetadata() {
 
     assert(PFS.MachineMetadataNodes[ID] == MD && "Tracking VH didn't work");
   } else {
-    if (PFS.MachineMetadataNodes.count(ID))
+    auto [It, Inserted] = PFS.MachineMetadataNodes.try_emplace(ID);
+    if (!Inserted)
       return error("Metadata id is already used");
-    PFS.MachineMetadataNodes[ID].reset(MD);
+    It->second.reset(MD);
   }
 
   return false;