There are some pipelines where we request SlotIndexes, but not
LiveIntervals before running PHIElimination. This happens in the MSP430
pipeline where StackColoring requests SlotIndexes, but nothing requests
LiveIntervals. Before PHIElimination would only update SlotIndexes
through LiveIntervals, which would mean we would fail to preserve
SlotIndexes if it was available but LiveIntervals was not. This would
cause crashes when later passes tried to use SlotIndexes under the
NewPM.
Reviewers: RKSimon, aeubanks, arsenm
Pull Request: https://github.com/llvm/llvm-project/pull/207074
