LiveIntervalAnalysis: cleanup addKills(), NFC

- Use more const modifiers
- Use references for things that can't be nullptr
- Improve some variable names

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224663 91177308-0d34-0410-b5e6-96231b3b80d8
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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224664 91177308-0d34-0410-b5e6-96231b3b80d8