[Attributor][FIX] Account for undef in the constant value lattice

The constant value lattice looks like this

```
  <None>
     |
  <undef>
  /  |   \
... <0>  ...
 \   |   /
 <unknown>
```
We did not account for the undef and assumed a value meant we could not
change anymore. Now we actually check if we have the same value as
before, which will signal CHANGED to the users when we go from undef to
a specific constant.

This fixes, among other things, the bug exposed by @ipccp4 in
`value-simplify.ll`.