[LV] Assert that there's a resume phi for epilogue loops (NFC).

This patch adds an assert to createAndCollectMergePhiForReduction to
make sure there is a resume phi when vectorizing the epilogue loop. This
is needed to set the resume value from the main vector loop.

This assertion guards against the issue caused the revert of
https://github.com/llvm/llvm-project/pull/78304.