Refactor debugging code, NFC.

Summary:
Refactor debugging routines to reduce code duplication. Remove a couple
of #include's that were not needed. Don't require MachineDominator as a
prereq for this pass (not needed).

These changes split off from http://reviews.llvm.org/D18827.

Reviewers: wmi, gbiv, qcolombet

Subscribers: llvm-commits, davidxl, jevinskie

Differential Revision: http://reviews.llvm.org/D18992

llvm-svn: 267766