Remove UdivSelf rewrite, add UdivZero rewrite

Remove UdivSelf rewrite, add UdivZero rewrite

This fixes bug 820. The issue was that (a udiv a) got rewriten to 1, which is
not correct when a is 0 (the result is all ones in that case). Even with the
--bv-div-zero-const flag disabled, the UdivSelf rewrite was incorrect because
it was applied to BITVECTOR_UDIV_TOTAL, which is "defined to be the all-ones
bit pattern, if divisor is 0" according to src/theory/bv/kinds . The commit
adds instead an optimization that returns all ones if the divisor of a
BITVECTOR_UDIV_TOTAL is zero.