Reland "[strings] Fix hash for exactly 512MB long strings"

This is a reland of 556f44c4

Original change's description:
> [strings] Fix hash for exactly 512MB long strings
> 
> Bug: chromium:1016237
> Change-Id: Idda1e44b5d578d1213aa54927ca68289bcdce8ac
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1878487
> Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
> Reviewed-by: Igor Sheludko <ishell@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#64552}

Bug: chromium:1016237
Change-Id: I92ff4da0b25877faddfd171105b77680f9e08037
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1918251Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65019}

src/strings/string-hasher-inl.h

@@ -33,9 +33,18 @@ uint32_t StringHasher::GetHashCore(uint32_t running_hash) {
 
 uint32_t StringHasher::GetTrivialHash(int length) {
   DCHECK_GT(length, String::kMaxHashCalcLength);
-  // String hash of a large string is simply the length.
-  return (static_cast<uint32_t>(length) << String::kHashShift) |
-         String::kIsNotArrayIndexMask | String::kIsNotIntegerIndexMask;
+  // The hash of a large string is simply computed from the length. We don't
+  // have quite enough bits, so we drop the least significant bit.
+  // TODO(9904): Free up one bit, so we don't have to drop anything here.
+  constexpr int kDroppedBits = 1;
+  // Ensure that the max length after dropping bits is small enough to be
+  // shifted without losing information.
+  STATIC_ASSERT(base::bits::CountLeadingZeros32(String::kMaxLength) +
+                    kDroppedBits >=
+                String::kHashShift);
+  uint32_t hash = static_cast<uint32_t>(length) >> kDroppedBits;
+  return (hash << String::kHashShift) | String::kIsNotArrayIndexMask |
+         String::kIsNotIntegerIndexMask;
 }
 
 template <typename schar>