MachineDominators: Define MachineDomTree type alias

This is a (very) small move towards making the machine dominators more
aligned with the IR dominators:

* DominatorTree / MachineDomTree is the class holding the dominator tree
* DominatorTreeWrapperPass / MachineDominatorTree is the corresponding
  (machine) function pass

This alignment will be used by analyses that are designed as templates
that work with LLVM IR as well as Machine IR.

Reviewed By: critson

Differential Revision: https://reviews.llvm.org/D112690