[optimzing] Fix codegen bug and improve type propagation

- don't bound the type if there are no relevant uses
- insert the bound type in the bounded block (this allows for condition
materialization without changing the logic there).
- add more comments
- add tests for BoundType generation
- fix GenerateTestAndBranch

Change-Id: I5c1fdda104da4a46775d207270220d410234a472