Fuzztest report in some simplification passes

... with a particular PRNG seed.

To reproduce:

```
bazel build -c opt //xls/passes:boolean_simplification_pass_test //xls/passes:concat_simplification_pass_test //xls/passes:unroll_pass_test
for f in bazel-bin/xls/passes/{boolean,concat}_simplification_pass_test bazel-bin/xls/passes/unroll_pass_test ; do FUZZTEST_PRNG_SEED=0XTqQUjm3mTR7_taTYGPr9w7_LQ8UvY41UvSCmwfe8k $f ; done

```

Results in crashes somewhere in `BoundedWidth()`:

```
F0728 05:40:20.782374 3869823 ir_fuzz_helpers.cc:394] Check failed: left_bound <= right_bound (1 vs. -9223372036854775808)

...
@ xls::BoundedWidth()
@ xls::GenIrNodesPass::HandleDecode()
@ xls::GenIrNodesPass::GenIrNodes()
@ xls::IrFuzzBuilder::BuildIr()
....
```