[Dominators] Simplify block and node printing

Summary:
This patch adds `BlockPrinter`-- a small wrapper for printing CFG nodes and DomTree nodes to `raw_ostream`. It is meant to be only used internally, for debugging and printing errors.

Reviewers: dberlin, sanjoy, grosser, davide

Reviewed By: grosser, davide

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D35286

llvm-svn: 308036