Add basic verification of LiveIntervals.

We verify that the LiveInterval is live at uses and defs, and that all
instructions have a SlotIndex.

Stuff we don't check yet:

- Is the LiveInterval minimal?
- Do all defs correspond to instructions or phis?
- Do all defs dominate all their live ranges?
- Are all live ranges continually reachable from their def?

llvm-svn: 110386