[fuzz] Crash in LLVM via eval_ir_main, target-independent node not lowered

**Describe the bug**

IR is here: [sample.ir.txt](https://github.com/user-attachments/files/18053493/sample.ir.txt)
Testevector is here: [testvector.pbtxt.txt](https://github.com/user-attachments/files/18053503/testvector.pbtxt.txt)

```
✓ /tmp/crashers-2024-12-07/95cce86e cdleary@vision 10:18:10
$ /home/cdleary/.cache/bazel/_bazel_cdleary/e3482f322694a8d1a6845914a1a3d787/execroot/com_google_xls/bazel-out/k8-dbg/bin/xls/tools/eval_ir_main --testvector_textproto=./testvector.pbtxt --use_llvm_jit ./sample.ir --logtostderr
t776: ch = <>
This target-independent node should have been selected!
UNREACHABLE executed at external/llvm-project/llvm/lib/CodeGen/SelectionDAG/InstrEmitter.cpp:1247!
Aborted
```

**Expected behavior**
No LLVM crash

**Environment (this can be helpful for troubleshooting):**
- OS: Ubuntu 24.04.1 LTS

**Additional context**
Was fuzzing overnight after #1774