LiveRangeEdit: Fix liveranges not shrinking on subrange kill.

If a dead instruction we may not only have a last-use in the main live
range but also in a subregister range if subregisters are tracked. We
need to partially rebuild live ranges in both cases.

The testcase only broke when subregister liveness was enabled. I
commited it in the current form because there is currently no flag to
enable/disable subregister liveness.

This fixes PR23720.

llvm-svn: 238785