CodeGen: Avoid iterator conversions in TwoAddressInstructionPass, NFC

Mostly through preferring MachineInstr&, avoid implicit conversions from
iterator to pointer.

Although this may bitrot (since there are other uses blocking me from
removing the implicit operator), this removes the last of the implicit
conversions from MachineInstrBundleIterator to MachineInstr* in the
LLVMCodeGen build target.

llvm-svn: 274893