Use getInterval() instead of getOrCreateInterval().

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175731 91177308-0d34-0410-b5e6-96231b3b80d8
Don't rely on the isDead() MachineOperand flag when updating LiveIntervals.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175732 91177308-0d34-0410-b5e6-96231b3b80d8