Avoid iterating with LiveIntervals::iterator.

Avoid iterating with LiveIntervals::iterator.

That is a DenseMap iterator keyed by pointers, so the iteration order is
nondeterministic.

I would like to replace the DenseMap with an IndexedMap which doesn't
allow iteration.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@158856 91177308-0d34-0410-b5e6-96231b3b80d8