Allow internal mutable array effect to be DCEd
Why? Previously we were being conservative about when we DCE effects.
Code like this:
```
@jax.jit
def f(x, y):
@jax.jit
def g(y):
y_ref = jax.new_ref(y)
y_ref[...] += 1
return y_ref[...]
_ = g(y)
return x
```
would *not* DCE the call to `g` despite it being functionally "pure"
(its Ref effects are entirely contained within the function and it is
the implementation of `lambda y: y + 1`) because the
InternalMutableArrayEffect would block DCE.
This change makes it so `g(y)` can get DCEd by allowing DCE to get rid
of equations that have internal mutable array effects if the outputs are
unused.
PiperOrigin-RevId: 867721260
