Track new virtual registers by register number.

Track new virtual registers by register number, rather than by the live
interval created for them. This is the first step in separating the
creation of new virtual registers and new live intervals.  Eventually
live intervals will be created and populated on demand after the virtual
registers have been created and used in instructions.
Notify LiveRangeEdit of new virtual registers.

Add a delegate class to MachineRegisterInfo with a single virtual
function, MRI_NoteNewVirtualRegister(). Update LiveRangeEdit to inherit
from this delegate class and override the definition of the callback
with an implementation that tracks the newly created virtual registers.
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.
Auto-compute live intervals on demand.

When new virtual registers are created during splitting/spilling, defer
creation of the live interval until we need to use the live interval.

Along with the recent commits to notify LiveRangeEdit when new virtual
registers are created, this makes it possible for functions like
TargetInstrInfo::loadRegFromStackSlot() and
TargetInstrInfo::storeRegToStackSlot() to create multiple virtual
registers as part of the process of generating loads/stores for
different register classes, and then have the live intervals for those
new registers computed when they are needed.