Replaced a bailout ID assertion with quadratic time complexity by a linear one.

Replaced a bailout ID assertion with quadratic time complexity by a linear one.

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.

Committed: http://code.google.com/p/v8/source/detail?r=13312

[Toon Verwaest, 2013-01-04 12:47:13]

lgtm