If X is known non-negative, that's still true if we fold the truncate
to create a smaller zext.
In the i128 tests, SelectionDAGBuilder aggressively truncates the
`zext nneg` to i64 to match `getShiftAmountTy`. If we don't preserve
the `nneg` we can't see that the shift amount argument being `signext`
means we don't need to do any extension
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