[Bennet] Conditional lazy generation

[ZippeyKeys12]

This fixes lazy generation for tutorial_queue.pass.c

Suppose we have the following.

let* x = lazy<pointer>();
let* y = OtherGen(x);
let* z = ... ;
instantiate(x) { ... };
long x := z;

If x is instantiated in OtherGen, that point can not be backtracked to, from x := z.
Therefore, we now keep x's generation eager, resulting in:

let* x = arbitrary<pointer>();
let* y = OtherGen(x);
let* z = ... ;
long x := z;