[Intl] Fix /k/i.test('\u212A')

Add logic stated in

Bug: v8:9731
Change-Id: I0b3468bbad11a178f36d682febd0e44214646de8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1828279
Commit-Queue: Frank Tang <ftang@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64027}

src/regexp/regexp-compiler.cc

@@ -736,7 +736,14 @@ static int GetCaseIndependentLetters(Isolate* isolate, uc16 character,
     CHECK(end - start + items <= letter_length);
     while (start <= end) {
       if (one_byte_subject && start > String::kMaxOneByteCharCode) break;
-      letters[items++] = (unibrow::uchar)(start);
+      // Only add to the output if character is not in ASCII range
+      // or the case equivalent character is in ASCII range.
+      // #sec-runtime-semantics-canonicalize-ch
+      // 3.g If the numeric value of ch ≥ 128 and the numeric value of cu < 128,
+      //     return ch.
+      if (!((start >= 128) && (character < 128))) {
+        letters[items++] = (unibrow::uchar)(start);
+      }
       start++;
     }
   }

test/intl/regress-9731.js

+// Copyright 2019 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+assertFalse(/k/i.test('\u212A'));
+assertTrue(/k/i.test('K'));
+assertTrue(/k/i.test('k'));
+
+assertFalse(/K/i.test('\u212A'));
+assertTrue(/K/i.test('K'));
+assertTrue(/K/i.test('k'));
+
+assertTrue(/\u212A/i.test('\u212A'));
+assertFalse(/\u212A/i.test('k'));
+assertFalse(/\u212A/i.test('K'));