nir: Do not use progress for unreachable code in return lowering.

We seem to use progress for two cases:
1) When we lowered some returns.
2) When we remove unreachable code.

If just case 2 happens we assert as state->return_flag has not
been allocated yet, but we are still trying to do insert all
predicates based on it.

This splits the concerns. We only use progress internally for case 1
and then keep track of 2 in a separate variable to indicate progress
in the return value of the pass.

This is slightly better than transforming the assert into
if (!state->return_flag) return, as the solution in this patch avoids
inserting predicates even if some other part of the might need them.

Fixes: 6e22ad6edc "nir: return early when lowering a return at the end of a function"
CC: 18.1 <mesa-stable@lists.freedesktop.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=106174
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>

src/compiler/nir/nir_lower_returns.c

@@ -37,6 +37,8 @@ struct lower_returns_state {
     * needs to be predicated on the return flag variable.
     */
    bool has_predicated_return;
+
+   bool removed_unreachable_code;
 };
 
 static bool lower_returns_in_cf_list(struct exec_list *cf_list,
@@ -162,8 +164,9 @@ lower_returns_in_block(nir_block *block, struct lower_returns_state *state)
           */
          return false;
       } else {
+         state->removed_unreachable_code = true;
          nir_cf_delete(&list);
-         return true;
+         return false;
       }
    }
 
@@ -262,9 +265,11 @@ nir_lower_returns_impl(nir_function_impl *impl)
    state.loop = NULL;
    state.return_flag = NULL;
    state.has_predicated_return = false;
+   state.removed_unreachable_code = false;
    nir_builder_init(&state.builder, impl);
 
    bool progress = lower_returns_in_cf_list(&impl->body, &state);
+   progress = progress || state.removed_unreachable_code;
 
    if (progress) {
       nir_metadata_preserve(impl, nir_metadata_none);