This reduces the time to run our test suite in debug mode considerably (from
8:43 to 4:05 on my local workstation using 32 threads). Note that the assertion
is so fast now that it doesn't need to be hidden behind --enable-slow-asserts.
Furthermore, the bookkeeping of the set is not measurable in all our benchmarks,
so I intentionally avoided any #ifdef chaos to keep things simple.

Review URL: https://codereview.chromium.org/11745027

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@13312 ce2b1a6d-e550-0410-aec6-3dcde31c8c00