[AArch64] Reenable the PBQP test now that the leak issue has been fixed.

David Blaikie's commits r217563 & r217564, which added shared_ptr to the
CostPool have fixed some memory leak issues exposed by the PBQP with
coalescing constraints.

The sanitizer bot was failing because of those leaks. Now that the leaks
are gone, we can reenable the aarch64/pbqp test.