dx: Remove a couple of hotspots

- Use concurrent hash map rather than hash map for interned
  RegisterSpec instances.
- Use bitmaps in SsaMethod.computeReachability()

Also use default concurrency level in concurrent collections.

Bug: 65231722
Test: dx/tests/run-all-tests
Change-Id: If117eef77215c07d7c7e033795adef7a44ce8805
6 files changed