LiveIntervalAnalysis: cleanup addKills(), NFC

- Use more const modifiers
- Use references for things that can't be nullptr
- Improve some variable names
LiveIntervalAnalysis: No kill flags for partially undefined uses.

We must not add kill flags when reading a vreg with some undefined
subregisters, if subreg liveness tracking is enabled.  This is because
the register allocator may reuse these undefined subregisters for other
values which are not killed.