Follow-up to 7f1262a322. That patch avoided removing the
call, but it still allowed the constant-folded result. This
makes the behavior consistent with 1-arg libm folding: if the
call potentially raises an exception, then we just bail out.
It seems likely that there are other corner-cases like this,
but the tests are incomplete, so we have lived with these
discrepancies for a long time. This was untested before the
the constant folding was expanded in D127964.
2981a94902