[SCEV] Special case sext in isKnownNonZero (#77834)

The existing logic in isKnownNonZero relies on unsigned ranges, which
can be problematic when our range calculation is imprecise. Consider the
following:
  %offset.nonzero = or i32 %offset, 1
  -->  %offset.nonzero U: [1,0) S: [1,0)
  %offset.i64 = sext i32 %offset.nonzero to i64
  -->  (sext i32 %offset.nonzero to i64) U: [-2147483648,2147483648)
                                         S: [-2147483648,2147483648)

Note that the unsigned range for the sext does contain zero in this case
despite the fact that it can never actually be zero.

Instead, we can push the query down one level - relying on the fact that
the sext is an invertible operation and that the result can only be zero
if the input is. We could likely generalize this reasoning for other
invertible operations, but special casing sext seems worthwhile.