Reduce memory usage of SSA Phi elimination and make it faster.

Use an ArenaBitVector instead of an ArenaSet<> that leaks
its allocated memory on clear(). We were also erroneously
using the O(n) helper ContainsElement() for the ArenaSet<>
instead of the O(log n) ArenaSet<>::find() which made the
methods with large number of processed Phis also very slow
to compile in addition to the enormous memory usage.

Bug: 28684584
Change-Id: Idc7604e51cfae643debd0378baf828a1430ec14c