Revert "Simplify IsValueType type check"

This property doesn't compile on all configurations in the PR build
validation.

src/System.Collections.Immutable/src/System/Collections/Immutable/ImmutableExtensions.cs

@@ -16,6 +16,21 @@ namespace System.Collections.Immutable
     /// </summary>
     internal static partial class ImmutableExtensions
     {
+        internal static bool IsValueType<T>()
+        {
+            if (default(T) != null)
+            {
+                return true;
+            }
+
+            Type t = typeof(T);
+            if (t.IsConstructedGenericType && t.GetGenericTypeDefinition() == typeof(Nullable<>))
+            {
+                return true;
+            }
+
+            return false;
+        }
 
 #if EqualsStructurally
 

src/System.Collections.Immutable/src/System/Collections/Immutable/ImmutableHashSet_1.cs

@@ -221,7 +221,7 @@ public ImmutableHashSet<T> Remove(T item)
         /// <param name="actualValue">The value from the set that the search found, or the original value if the search yielded no match.</param>
         /// <returns>A value indicating whether the search was successful.</returns>
         /// <remarks>
-        /// This can be useful when you want to reuse a previously stored reference instead of 
+        /// This can be useful when you want to reuse a previously stored reference instead of
         /// a newly constructed one (so that more sharing of references can occur) or to look up
         /// a value that has more complete data than the value you currently have, although their
         /// comparer functions indicate they are equal.
@@ -1014,7 +1014,7 @@ private static ImmutableHashSet<T> Wrap(SortedInt32KeyNode<HashBucket> root, IEq
         /// <returns>The equality comparer to use.</returns>
         private static IEqualityComparer<HashBucket> GetHashBucketEqualityComparer(IEqualityComparer<T> valueComparer)
         {
-            if (!typeof(T).IsValueType)
+            if (!ImmutableExtensions.IsValueType<T>())
             {
                 return HashBucketByRefEqualityComparer.DefaultInstance;
             }