aco/insert_exec: reset top exec for p_discard_if

Reviewed-by: Timur Kristóf <timur.kristof@gmail.com>

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/12363
Fixes: 31f62a6123 ("aco/insert_exec: don't always reset top exec")
(cherry picked from commit 272ff275fa)

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/32852>

.pick_status.json

@@ -4,7 +4,7 @@
         "description": "aco/insert_exec: reset top exec for p_discard_if",
         "nominated": true,
         "nomination_type": 2,
-        "resolution": 0,
+        "resolution": 3,
         "main_sha": null,
         "because_sha": "31f62a6123d46311bed7a54dc10e518bbe4c5f8d",
         "notes": null

src/amd/compiler/aco_insert_exec_mask.cpp

@@ -467,6 +467,7 @@ process_instructions(exec_ctx& ctx, Block* block, std::vector<aco_ptr<Instructio
             exit_cond = andn2->definitions[1].getTemp();
          }
 
+         info.exec.back().op = Operand(exec, bld.lm);
          instr->opcode = aco_opcode::p_exit_early_if;
          instr->operands[0] = bld.scc(exit_cond);
          assert(!ctx.handle_wqm || (info.exec[0].type & mask_type_wqm) == 0);