RyuJIT: Don't emit cast helpers for (T)array.Clone() (#45311)

Note that Array.Clone()'s return type is the same as the type of its first operand.

This allows optimization of casts on the result of calls to Clone().

src/coreclr/jit/gentree.cpp

@@ -17757,6 +17757,14 @@ CORINFO_CLASS_HANDLE Compiler::gtGetClassHandle(GenTree* tree, bool* pIsExact, b
         case GT_CALL:
         {
             GenTreeCall* call = tree->AsCall();
+            if (call->gtFlags & CORINFO_FLG_JIT_INTRINSIC)
+            {
+                if (lookupNamedIntrinsic(call->gtCallMethHnd) == NI_System_Array_Clone)
+                {
+                    objClass = gtGetClassHandle(call->gtCallThisArg->GetNode(), pIsExact, pIsNonNull);
+                    break;
+                }
+            }
             if (call->IsInlineCandidate())
             {
                 // For inline candidates, we've already cached the return

src/coreclr/jit/importer.cpp

@@ -4809,6 +4809,13 @@ NamedIntrinsic Compiler::lookupNamedIntrinsic(CORINFO_METHOD_HANDLE method)
                 result = NI_System_GC_KeepAlive;
             }
         }
+        else if (strcmp(className, "Array") == 0)
+        {
+            if (strcmp(methodName, "Clone") == 0)
+            {
+                result = NI_System_Array_Clone;
+            }
+        }
         else if (strcmp(className, "Type") == 0)
         {
             if (strcmp(methodName, "get_IsValueType") == 0)

src/coreclr/jit/namedintrinsiclist.h

@@ -43,6 +43,7 @@ enum NamedIntrinsic : unsigned short
     NI_System_Type_get_IsValueType,
     NI_System_Type_IsAssignableFrom,
     NI_System_Type_IsAssignableTo,
+    NI_System_Array_Clone,
 
     // These are used by HWIntrinsics but are defined more generally
     // to allow dead code optimization and handle the recursion case

src/libraries/System.Private.CoreLib/src/System/Array.cs

@@ -319,6 +319,7 @@ void IList.RemoveAt(int index)
 
         // Make a new array which is a shallow copy of the original array.
         //
+        [Intrinsic]
         public object Clone()
         {
             return MemberwiseClone();