This cleans up code and is more in line with the general philosophy of
modifying LiveIntervals through LiveIntervalAnalysis instead of changing
them directly.
This also fixes a case where SplitEditor::removeBackCopies() would miss
the subregister ranges.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@226690 91177308-0d34-0410-b5e6-96231b3b80d8
