Fix the logic in ExprSMTLIBPrinter::getSort

This now corresponds to the sorts of the operations we emit, as far as I
can tell.

Read is one example of an operation that now works correctly (with 1-bit
array ranges).

It's also possible (but not very useful, and I don't think KLEE can
produce it) for other operations such as Add to operate on 1-bit values,
and this patch also fixes those.