This implements support for isKnownNonZero, computeKnownBits when freeze is involved.
br (x != 0), BB1, BB2 BB1: y = freeze x
In the above program, we can say that y is non-zero. The reason is as follows:
(1) If x was poison, br (x != 0) raised UB
(2) If x was fully undef, the branch again raised UB
(3) If x was non-zero partially undef, say undef | 1, freeze x will return a nondeterministic value which is also non-zero.
(4) If x was just a concrete value, it is trivial