LiveIntervalAnalysis: Fix performance bug that I introduced in r224663.

Without a reference the code did not remember when moving the iterators
of the subranges/registerunit ranges forward and instead would scan from
the beginning again at the next position.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224803 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed