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>

src/compiler/nir/nir_opt_loop_unroll.c

@@ -575,9 +575,17 @@ nir_opt_loop_unroll_impl(nir_function_impl *impl,
                                 &has_nested_loop);
    }
 
-   if (progress)
+   if (progress) {
       nir_lower_regs_to_ssa_impl(impl);
 
+      /* Calling nir_convert_loop_to_lcssa() adds extra phi nodes which may
+       * not be valid if they're used for something such as a deref.
+       *  Remove any unneeded phis.
+       */
+      nir_copy_prop(impl->function->shader);
+      nir_opt_remove_phis_impl(impl);
+   }
+
    return progress;
 }