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