Fuzzer crash c1e3

Repro:

```
bazel test -c opt //xls/fuzzer:run_crasher_test_2025-09-03_c1e3
```

Error message:

```
Error: UNIMPLEMENTED: Proc combinational generator only supports streaming output channels which can be determined to be mutually exclusive, got 2 output channels which were not proven to be mutually exclusive

Error: INTERNAL: /usr/local/google/home/davidplass/.cache/bazel/_bazel_davidplass/7af3f84567cca338af87a04c2463db99/sandbox/linux-sandbox/7175/execroot/_main/bazel-out/k8-opt/bin/xls/fuzzer/run_crasher_test_2025-09-03_c1e3.runfiles/_main/xls/tools/codegen_main returned a non-zero exit status (1): /usr/local/google/home/davidplass/.cache/bazel/_bazel_davidplass/7af3f84567cca338af87a04c2463db99/sandbox/linux-sandbox/7175/execroot/_main/bazel-out/k8-opt/bin/xls/fuzzer/run_crasher_test_2025-09-03_c1e3.runfiles/_main/xls/tools/codegen_main --output_signature_path=module_sig.textproto --delay_model=unit --nouse_system_verilog --output_block_ir_path=sample.block.ir --generator=combinational --reset_data_path=false /tmp/temp_directory_quYVZC/sample.opt.ir --logtostderr

Subprocess stderr:
Error: UNIMPLEMENTED: Proc combinational generator only supports streaming output channels which can be determined to be mutually exclusive, got 2 output channels which were not proven to be mutually exclusive
```