Remove LiveIntervals::iterator.

Live intervals for regunits and virtual registers are stored separately,
and physreg live intervals are going away.

To visit the live ranges of all virtual registers, use this pattern
instead:

  for (unsigned i = 0, e = MRI->getNumVirtRegs(); i != e; ++i) {
    unsigned Reg = TargetRegisterInfo::index2VirtReg(i);
    if (MRI->reg_nodbg_empty(Reg))
      continue;

llvm-svn: 158879