Add the MachineInstrSpan class.

MachineInstrSpan is initialized with a MachineBasicBlock::iterator,
and is intended to track which instructions are inserted before/after
that instruction from the time the MachineInstrSpan is created.

It provides a begin()/end() interface to walk the range of
instructions inserted around the initial instruction (including that
initial instruction).

It also provides a getInitial() interface to return the initial
iterator.

llvm-svn: 188436