MachineRegionInfo: Fix pass initialization

- Adapt MachineBasicBlock::getName() to have the same behavior as the IR
  BasicBlock (Value::getName()).
- Add it to lib/CodeGen/CodeGen.cpp::initializeCodeGen so that it is linked in
  the CodeGen library.
- MachineRegionInfoPass's name conflicts with RegionInfoPass's name ("region").
- MachineRegionInfo should depend on MachineDominatorTree,
  MachinePostDominatorTree and MachineDominanceFrontier instead of their
  respective IR versions.
- Since there were no tests for this, add a X86 MIR test.

Patch by Francis Visoiu Mistrih<fvisoiumistrih@apple.com>

llvm-svn: 295518