[ValueTracking] isKnownNonZero, computeKnownBits for freeze

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

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D75808