FLAG_disable_write_barriers is a constexpr so the V8_LIKELY macro isn't
necessary. Interestingly, it can also cause clang to warn that the code
is unreachable, whereas without `__builtin_expect()` the compiler
doesn't mind. See for example:

```
constexpr bool kNo = false;

void warns() {
  if (__builtin_expect(kNo, 0)) {
    int a = 42;
  }
}

void does_not_warn() {
  if (kNo) {
    int a = 42;
  }
}
```

Compiling V8 for arm64 with both `v8_disable_write_barriers = true` and
`v8_enable_pointer_compression = false` would trigger this warning.

Bug: v8:9533
Change-Id: Id2ae156d60217007bb9ebf50628e8908e0193d05
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2534811Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Pierre Langlois <pierre.langlois@arm.com>
Cr-Commit-Position: refs/heads/master@{#71157}