Add a feature flag to not use GVM in Linq Select

src/libraries/System.Linq/src/System/Linq/Select.cs

@@ -25,7 +25,19 @@ public static IEnumerable<TResult> Select<TSource, TResult>(
 
             if (source is Iterator<TSource> iterator)
             {
-                return iterator.Select(selector);
+                // With native AOT, calling into the `Select` generic virtual method results in NxM
+                // expansion of native code. If the option is enabled, we don't call the generic virtual
+                // for value types. We don't do the same for reference types because reference type
+                // expansion can happen lazily at runtime and the AOT compiler does postpone it (we
+                // don't need more code, just more data structures describing the new types).
+                if (IsSizeOptimized && typeof(TResult).IsValueType)
+                {
+                    return new IEnumerableSelectIterator<TSource, TResult>(iterator, selector);
+                }
+                else
+                {
+                    return iterator.Select(selector);
+                }
             }
 
             if (source is IList<TSource> ilist)