ir_conv: chan array cannot be created from from existing chans

**Describe the bug**
Creating a chan array from an existing set of chans seems to crash `ir_converter_main`
```
[throw_delegate.cc:112] RAW: absl::container_internal::raw_hash_map<>::at
absl::raw_log_internal::RawLog()
absl::base_internal::ThrowStdOutOfRange()
xls::dslx::ProcConfigIrConverter::Finalize()
xls::dslx::(anonymous namespace)::ConvertCallGraph()
xls::dslx::ConvertOneFunctionIntoPackageInternal<>()
xls::dslx::ConvertOneFunctionIntoPackage()
xls::dslx::ConvertFilesToPackage()
main
__libc_start_main
_start
```

**To Reproduce**
```
proc SomeProc {
some_chan_array: chan[4] in;

config(some_chan_0: chan in, some_chan_1: chan in, some_chan_2: chan in,
some_chan_3: chan in) {
([some_chan_0, some_chan_1, some_chan_2, some_chan_3],)
}

init { }

next(state: ()) { }
}
```