[LoopDeletion] Clear SCEV loop dispositions

`Loop::makeLoopInvariant` can hoist instructions out of loops, so loop
dispositions for the loop it operated on may need to be cleared.  We can
be smarter here (especially around how `forgetLoopDispositions` is
implemented), but let's be correct first.

Fixes PR27570.

llvm-svn: 268406