nir/opt_loop_unroll: Remove unneeded phis if we make progress

nir/opt_loop_unroll: Remove unneeded phis if we make progress

Now that SSA values can be derefs and they have special rules, we have
to be a bit more careful about our LCSSA phis.  In particular, we need
to clean up in case LCSSA ended up creating a phi node for a deref.
This avoids validation issues with some CTS tests with the following
patch, but its possible this we could also see the same problem with
the existing unrolling passes.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>