Revert "[LiveInterval] Simplify. NFC"

This reverts commit e733b80f.

This caused a major compile-time regression:
https://llvm-compile-time-tracker.com/compare.php?from=3b7c3a654c9175f41ac871a937cbcae73dfb3c5d&to=e733b80f3cba26bf2df9bd691120f37fc1af21ce&stat=instructions

About 1% on average, 10% on individual files.