Revise dataflow to use middle::cfg flowgraph to drive analysis.

Fix #6298.

This also includes some drive by fixes for some other issues, noted in the commits.

I still need to integrate regression tests for some cases that I noticed were missing from our unit test suite (i.e. things that compiling rustc exposes that should have been exposed when doing `make check-stage1`).  So do not land this yet, until I get the chance to add those tests.

I just wanted to get the review process started soon, since this has been long in the coming.