Thread safety analysis: Eliminate parameter from intersectAndWarn (NFC)
    
    We were modifying precisely when intersecting the lock sets of multiple
    predecessors without back edge. That's no coincidence: we can't modify
    on back edges, it doesn't make sense to modify at the end of a function,
    and otherwise we always want to intersect on forward edges, because we
    can build a new lock set for those.
    
    Reviewed By: aaron.ballman
    
    Differential Revision: https://reviews.llvm.org/D101755