[vm/compiler] Do not convert null-aware int? equality to IfThenElse

IfThenElse instruction expects a simple Smi comparison and it doesn't
support null-aware int? comparison (although it uses the same kTagged
representation).

TEST=runtime/tests/vm/dart/regress_b378737064_test.dart
Fixes b/378737064

Change-Id: Iaf9243ff5505b986bbfe4510834972e75869dc61
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/394570
Commit-Queue: Alexander Markov <alexmarkov@google.com>
Reviewed-by: Ryan Macnak <rmacnak@google.com>

runtime/tests/vm/dart/regress_b378737064_test.dart

@@ -0,0 +1,22 @@
+// Copyright (c) 2024, 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.
+
+// Regression test for b/378737064.
+//
+// Verifies that compiler doesn't crash after it incorrectly converts
+// null-aware int? comparison to IfThenElse.
+
+import 'package:expect/expect.dart';
+
+@pragma('vm:never-inline')
+int foo(int? x, int? y) => x == y ? 0 : 255;
+
+main() {
+  Expect.equals(0, foo(0, 0));
+  Expect.equals(255, foo(0, 42));
+  Expect.equals(0, foo(null, null));
+  Expect.equals(255, foo(null, 0x1234567890));
+  Expect.equals(0, foo(0x1234567890, 0x1234567890));
+  Expect.equals(255, foo(0x1234567890, 0x1234567891));
+}

runtime/vm/compiler/backend/il.cc

@@ -6564,7 +6564,12 @@ bool IfThenElseInstr::Supports(ConditionInstr* condition,
     // by if-conversion.
     return !strict_compare->needs_number_check();
   }
-  if (auto* comparison = condition->AsComparison()) {
+  if (auto* equality = condition->AsEqualityCompare()) {
+    // Non-smi comparisons are not supported by if-conversion.
+    return (equality->input_representation() == kTagged) &&
+           !equality->is_null_aware();
+  }
+  if (auto* comparison = condition->AsRelationalOp()) {
     // Non-smi comparisons are not supported by if-conversion.
     return comparison->input_representation() == kTagged;
   }