Avoid excessive allocation of std::set<> nodes in SwapSpace.

This does not affect the overall memory usage but avoids
a lot of deallocations followed by allocation.

Measured compilation of a big app using heap track:
  bytes allocated in total (ignoring deallocations): 4.14GB -> 4.04GB
  calls to allocation functions: 21662554 -> 19545823

Test: m test-art-host-gtest
Test: Manually check that oat file for the big app remains identical.
Bug: 34053922
Change-Id: I00568422ba5510550986e29f30bace9ae6245269