[PostDom] document the current handling of infinite loops and unreachables

Summary:
As we are in the process of changing the behavior of how the post-dominator tree
is computed, make sure we have some more test coverage in this area.

Current inconsistencies:

  - Newly unreachable nodes are not added as new roots, in case the PDT is updated
    but not rebuilt.

  - Newly unreachable loops are not added to the CFG at all (neither when
    building from scratch nor when updating the CFG). This is inconsistent with
    the fact that unreachables are added to the PDT, but unreachable loops not.
    On the other side, PDT relationships are not loosened at the moment in
    cases where new unreachable loops are built.

This commit is providing additional test coverage for
https://reviews.llvm.org/D35851

Reviewers: dberlin, kuhar

Reviewed By: kuhar

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D36107

llvm-svn: 309684