aco/insert_exec_mask: set Exact mode after p_discard_if when necessary

Fixes: 5e9df85b1a ('aco: optimize discard_if when WQM is not needed afterwards')
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25038>

src/amd/compiler/aco_insert_exec_mask.cpp

@@ -570,6 +570,7 @@ process_instructions(exec_ctx& ctx, Block* block, std::vector<aco_ptr<Instructio
                       (mask_type_exact | mask_type_global));
                current_exec = get_exec_op(ctx.info[block->index].exec.back().first);
                ctx.info[block->index].exec[0].first = Operand(bld.lm);
+               state = Exact;
             }
          }