r600/sfn: don't propagate registers into conditional test

We don't check whether the register is overwritten between the actual
conditional test and the test of the used result, so don't try to
optimize the evaluation of the conditional.

Fixes: 79ca456b48
   r600/sfn: rewrite NIR backend

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18983>
(cherry picked from commit 6de40d17ba)

.pick_status.json

@@ -1012,7 +1012,7 @@
         "description": "r600/sfn: don't propagate registers into conditional test",
         "nominated": true,
         "nomination_type": 1,
-        "resolution": 0,
+        "resolution": 1,
         "main_sha": null,
         "because_sha": "79ca456b4837b3bc21cf9ef3c03c505c4b4909f6"
     },

src/gallium/drivers/r600/sfn/sfn_peephole.cpp

@@ -212,11 +212,22 @@ void ReplaceIfPredicate::visit(AluInstr *alu)
    if (new_op == op0_nop)
       return;
 
-   /* Have to figure out how to pass the dependency correctly */
-   /*for (auto& s : alu->sources()) {
-      if (s->as_register() && s->as_register()->addr())
+   for (auto& s : alu->sources()) {
+      auto reg = s->as_register();
+      /* Protext against propagating
+       *
+       *   V = COND(R, X)
+       *   R = SOME_OP
+       *   IF (V)
+       *
+       * to
+       *
+       *   R = SOME_OP
+       *   IF (COND(R, X))
+       */
+      if (reg && !reg->is_ssa())
          return;
-   }*/
+   }
 
    m_pred->set_op(new_op);
    m_pred->set_sources(alu->sources());