Fix and improve static evaluation of constant expressions.

- Fix the definition of art::HSub::Evaluate.
- Qualify Evaluate methods as OVERRIDE.
- Evaluate comparisons in a deterministic way: if a
  comparison is true, always return 1 (instead of letting
  the compiler return any non-null value).
- Better exercise static evaluation of constant expressions
  in compiler/optimizing/constant_propagation_test.cc.

Change-Id: I13d0862e5f4eba1275016fb8c3c17e9aff54408b