Improve data structure use in RemoveUnneededTries

1) The new/malloc were taking an important part of the runtime. By
   using scope allocator on local compiles, the method is now ~33%
   faster.

2) Use BitVectors instead of sets in TryBelongingInformation which
   avoids tree rebalancing after set merging. This improves the
   method a further ~20%, for a total of ~55% combined with 1).

Test: art/test/testrunner/testrunner.py --host --64 --optimizing -b
Change-Id: If9ad005d0d6f89656ba5aa0e37e3061af272ba07