[BPI] Refactor post domination calculation and simple fix for ColdCall

Collection of PostDominatedByUnreachable and PostDominatedByColdCall have been
split out of heuristics itself. Update of the data happens now for each basic
block (before update for PostDominatedByColdCall might be skipped if
unreachable or matadata heuristic handled this basic block).

This separation allows re-ordering of heuristics without loosing
the post-domination information.

Reviewers: sanjoy, junbuml, vsk, chandlerc, reames

Reviewed By: chandlerc

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D31701

llvm-svn: 300029