arm64: Remove widening casts before truncating

src/coreclr/jit/morph.cpp

@@ -8640,6 +8640,19 @@ GenTree* Compiler::fgOptimizeCast(GenTreeCast* cast)
 
         var_types castToType = cast->CastToType();
 
+        // For small-int casts fed by a widening int->long, remove the widening so we truncate directly
+        // from the original int value.
+        if (varTypeIsSmall(castToType) && src->OperIs(GT_CAST) && !src->gtOverflow())
+        {
+            GenTreeCast* widening = src->AsCast();
+            if (varTypeIsLong(widening->CastToType()) && (genActualType(widening->CastFromType()) == TYP_INT))
+            {
+                cast->CastOp() = widening->CastOp();
+                DEBUG_DESTROY_NODE(widening);
+                src = cast->CastOp();
+            }
+        }
+
         // For indir-like nodes, we may be able to change their type to satisfy (and discard) the cast.
         if (varTypeIsSmall(castToType) && (genTypeSize(castToType) == genTypeSize(src)) &&
             src->OperIs(GT_IND, GT_LCL_FLD))

src/tests/Common/CoreCLRTestLibrary/CoreCLRTestLibrary.csproj

@@ -7,6 +7,7 @@
   </PropertyGroup>
   <ItemGroup>
     <Compile Include="AssertExtensions.cs" />
+    <Compile Include="Expect.cs" />
     <Compile Include="Generator.cs" />
     <Compile Include="Logging.cs" />
     <Compile Include="TestFramework.cs" />

src/tests/Common/CoreCLRTestLibrary/Expect.cs

@@ -0,0 +1,41 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+using System.Linq.Expressions;
+using Xunit;
+
+namespace TestLibrary;
+
+public static class Expect
+{
+    public static void ExpectEqual<T>(Expression<Func<T>> expr, T expected, ref bool fail)
+    {
+        var compiled = expr.Compile();
+        T actual = compiled();
+
+        // Get just the expression body text
+        string exprText = expr.Body.ToString();
+
+        if (!Equals(actual, expected))
+        {
+            Console.WriteLine($"{exprText} = {actual}, expected {expected}");
+            fail = true;
+        }
+    }
+
+    public static void ExpectTrue(Expression<Func<bool>> expr, ref bool fail)
+    {
+        var compiled = expr.Compile();
+        bool actual = compiled();
+
+        // Get just the expression body text
+        string exprText = expr.Body.ToString();
+
+        if (!actual)
+        {
+            Console.WriteLine($"{exprText} = {actual}, expected True");
+            fail = true;
+        }
+    }
+}