[ExecutionEngine] Simplify code with DenseMap::operator[] (NFC) (llvm#115115)

Author: kazutakahirata

llvm/include/llvm/ExecutionEngine/JITLink/aarch32.h

@@ -394,7 +394,7 @@ class StubsManager_v7 {
   using StubMapEntry = std::tuple<Symbol *, Symbol *>;
 
   Symbol *&getStubSymbolSlot(StringRef Name, bool Thumb) {
-    StubMapEntry &Stubs = StubMap.try_emplace(Name).first->second;
+    StubMapEntry &Stubs = StubMap[Name];
     if (Thumb)
       return std::get<1>(Stubs);
     return std::get<0>(Stubs);