Only create dictionary when analysis is enabled.

src/Compilers/CSharp/Portable/Compilation/MemberSemanticModel.cs

@@ -73,7 +73,10 @@ internal abstract partial class MemberSemanticModel : CSharpSemanticModel
             _speculatedPosition = speculatedPosition;
 
             _operationFactory = new Lazy<CSharpOperationFactory>(() => new CSharpOperationFactory(this));
-            _analyzedVariableTypesOpt = new ConcurrentDictionary<LocalSymbol, LocalSymbol>();
+            if (Compilation.NullableSemanticAnalysisEnabled)
+            {
+                _analyzedVariableTypesOpt = new ConcurrentDictionary<LocalSymbol, LocalSymbol>();
+            }
         }
 
         public override CSharpCompilation Compilation