Rename LiveRange to LiveInterval::Segment

The Segment struct contains a single interval; multiple instances of this struct
are used to construct a live range, but the struct is not a live range by
itself.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@192392 91177308-0d34-0410-b5e6-96231b3b80d8
16 files changed