[vm,compiler] Fix handling of the argument of _simpleInstanceOf

The 2nd argument of _simpleInstanceOf call is always a
Constant (type). However, a Redefinition can be inserted in the middle.

So, instead of ArgumentAt(1)->AsConstant()->value() it is
more safe to use ArgumentValueAt(1)->BoundConstant() as
BindsToConstant/BoundConstant unwraps Redefinition(s) via
OriginalDefinition().

TEST=runtime/tests/vm/dart/regress_63211_test.dart
Fixes https://github.com/dart-lang/sdk/issues/63211

Change-Id: Ie4a473ebe2deee8562e6634a792f02b0dcefc918
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/497761
Reviewed-by: Slava Egorov <vegorov@google.com>
Commit-Queue: Alexander Markov <alexmarkov@google.com>

runtime/tests/vm/dart/regress_63211_test.dart

@@ -0,0 +1,51 @@
+// Copyright (c) 2026, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// Verify that 'is Foo' is handled correctly when constant Foo is
+// hidden by Redefinition.
+// Regression test for https://github.com/dart-lang/sdk/issues/63211.
+
+// VMOptions=--optimization-counter-threshold=100 --no-background-compilation
+
+import 'package:expect/expect.dart';
+
+class Foo1 {
+  Foo1(this.x);
+  int x;
+
+  @pragma('vm:prefer-inline')
+  @override
+  bool operator ==(Object other) {
+    return other is Foo1 && x == other.x;
+  }
+}
+
+final v1 = Foo1(42);
+const c1 = Foo1;
+
+void test1() {
+  Expect.isFalse(v1 == c1);
+}
+
+class Foo2 {
+  @pragma('vm:prefer-inline')
+  @override
+  bool operator ==(Object other) {
+    return other is Foo2;
+  }
+}
+
+final v2 = Foo2();
+const c2 = Foo2;
+
+void test2() {
+  Expect.isFalse(v2 == c2);
+}
+
+void main() {
+  for (int i = 0; i < 200; ++i) {
+    test1();
+    test2();
+  }
+}