Teach LiveIntervals how to verify themselves and start using it in some

Teach LiveIntervals how to verify themselves and start using it in some
of the trick merge routines. This adds a layer of testing that was
necessary when implementing more efficient (and complex) merge logic for
this datastructure.

No functionality changed here.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@159981 91177308-0d34-0410-b5e6-96231b3b80d8