[LoopIdiom] 'arithmetic right-shift until zero': don't turn potentially infinite loops into finite ones

Nowadays LLVM does not assume that all loops are finite,
so if we want to produce a finite loop from a potentially-infinite one,
we must ensure that the original loop is known to be a finite one.

For this transform, it only matters for arithmetic right-shifts.
For them, either the function or the loop must be known to
be `mustprogress`, or the original value being shifted must be known
to be non-negative (because iff the sign bit was set,
it will never become zero, but will become `-1` in the "end").

It would be really good for alive2 to actually complain about this,
but it currently does not: https://github.com/AliveToolkit/alive2/issues/726

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