Revert "LiveIntervalAnalysis: Rework constructMainRangeFromSubranges()"

This reverts commit r269016 and also the follow-up commit r269020.

This patch caused PR27705.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@269344 91177308-0d34-0410-b5e6-96231b3b80d8
7 files changed