Remove duplicate GenericCache getter call, revert to direct throw

Co-authored-by: Jan Kotas <jkotas@microsoft.com>

Author: mikernet

src/coreclr/System.Private.CoreLib/src/System/Runtime/CompilerServices/RuntimeHelpers.CoreCLR.cs

@@ -188,8 +188,7 @@ public static object GetUninitializedObject(
             if (type is not RuntimeType rt)
             {
                 ArgumentNullException.ThrowIfNull(type);
-                static void Throw(Type type) => throw new SerializationException(SR.Format(SR.Serialization_InvalidType, type));
-                Throw(type);
+                throw new SerializationException(SR.Format(SR.Serialization_InvalidType, type));
             }
 
             return rt.GetUninitializedObject();

src/coreclr/System.Private.CoreLib/src/System/RuntimeType.CoreCLR.cs

@@ -3886,9 +3886,11 @@ private void CreateInstanceCheckThis()
         [DebuggerHidden]
         internal object GetUninitializedObject()
         {
-            if (GenericCache is not CreateUninitializedCache cache)
+            object? genericCache = GenericCache;
+
+            if (genericCache is not CreateUninitializedCache cache)
             {
-                if (GenericCache is ActivatorCache activatorCache)
+                if (genericCache is ActivatorCache activatorCache)
                 {
                     cache = activatorCache.GetCreateUninitializedCache(this);
                 }