[Attributor][FIX] Do not introduce multiple instances of SSA values

If we have a recursive function we could create multiple instantiations
of an SSA value, one per recursive invocation of the function. This is a
problem as we use SSA value equality in various places. The basic idea
follows from this test:

```
static int r(int c, int *a) {
  int X;
  return c ? r(false, &X) : a == &X;
}

int test(int c) {
  return r(c, undef);
}
```

If we look through the argument `a` we will end up with `X`. Using SSA
value equality we will fold `a == &X` to true and return true even
though it should have been false because `a` and `&X` are from different
instantiations of the function.

Various tests for this  have been placed in value-simplify-instances.ll
and this commit fixes them all by avoiding to produce simplified values
that could be non-unique at runtime. Thus, the result of a simplify
value call will always be unique at runtime or the original value, both
do not allow to accidentally compare two instances of a value with each
other and conclude they are equal statically (pointer equivalence) while
they are unequal at runtime.