CodeGen: Add an explicit BuildMI overload for MachineInstr&

Add an explicit overload to BuildMI for MachineInstr& to deal with
insertions inside of instruction bundles.

- Use it to re-implement MachineInstr* to give it coverage.

- Document how the overload for MachineBasicBlock::instr_iterator
  differs from that for MachineBasicBlock::iterator (the previous
  (implicit) overload for MachineInstr&).

- Add a comment explaining why the MachineInstr& and MachineInstr*
  overloads don't universally forward to the
  MachineBasicBlock::instr_iterator overload.

Thanks to Justin for noticing the API quirk.  While this doesn't fix any
known bugs -- all uses of BuildMI with a MachineInstr& were previously
using MachineBasicBlock::iterator -- it protects against future bugs.

llvm-svn: 274193