Revert "[regalloc] Ensure Query::collectInterferringVregs is called before interval iteration"

This reverts commit d40b4911.

This causes a large compile-time regression:
https://llvm-compile-time-tracker.com/compare.php?from=0aa637b2037d882ddf7861284169abf63f524677&to=d40b4911bd9aca0573752e065f29ddd9aff280e1&stat=instructions