Remove dead function.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185731 91177308-0d34-0410-b5e6-96231b3b80d8
Use modern API to avoid exposing LiveInterval internals.

No functional change intended.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185733 91177308-0d34-0410-b5e6-96231b3b80d8