out of range internal error when passing non-channel array to spawn

**Describe the bug**

ir_convert throws an internal error when passing a non-channel to spawn, see the snippets and error below.

**To Reproduce**

```
spawn matmul(
activations, results,
F32[4][4]:[
[F32_2, F32_0, F32_0, F32_0], [F32_0, F32_2, F32_0, F32_0],
[F32_0, F32_0, F32_2, F32_0], [F32_0, F32_0, F32_0, F32_2],
]);
```

```
F0724 08:23:23.286113 10325 throw_delegate.cc:112] RAW: absl::container_internal::raw_hash_map<>::at
@ 0x558387e40271 absl::raw_log_internal::RawLog()
@ 0x558387e3f2a6 absl::base_internal::ThrowStdOutOfRange()
@ 0x5583843f9684 absl::container_internal::raw_hash_map<>::at<>()
@ 0x5583843fc8ef xls::dslx::ProcConfigIrConverter::HandleSpawn()
@ 0x5583843f995e xls::dslx::ProcConfigIrConverter::HandleStatement()
@ 0x5583843f9700 xls::dslx::ProcConfigIrConverter::HandleStatementBlock()
@ 0x5583843fa324 xls::dslx::ProcConfigIrConverter::HandleFunction()
@ 0x55838439fa98 xls::dslx::(anonymous namespace)::ConvertCallGraph()
@ 0x55838439dff3 xls::dslx::ConvertModuleIntoPackage()
@ 0x5583843a0a5a xls::dslx::ConvertModuleToPackage()
@ 0x55838438e546 xls::dslx::AbstractTestRunner::ParseAndTest()
@ 0x55838435917d main
@ 0x7f29ae1963d4 __libc_start_main
@ 0x55838435702a _start
```