can call send/recv from helper fn

it seems that `send` and `recv` operation fails when being called from helper `fn`s invoked from a `proc` (even if being passed the appropriate `token` and `chan`):
```
output: chan out;

init {
u8:0
}

config(output: chan out) {
(output,)
}

next(tok: token, state: u8) {
let tok = send(tok, output, state);
state + u8:1
}
}

fn assert_recv_eq(tok: token, output_r: chan in, expected: u8) -> token {
let (tok, value) = recv(tok, output_r);
let _ = assert_eq(value, expected);
tok
}

#[test_proc]
proc counter_test {
terminator: chan out;
output_s: chan out;
output_r: chan in;

init {
()
}

config(t: chan out) {
let (output_s, output_r) = chan;
spawn counter(output_s);
(t, output_s, output_r)
}

next(tok: token, state: ()) {
let tok = assert_recv_eq(tok, output_r, u8:0);
let tok = send(tok, terminator, true);
()
}
}
```
would produce the following error:
```
[ RUN UNITTEST ] counter_test
E0502 10:10:52.469354 31692 bytecode_emitter.cc:1088] INTERNAL: XLS_RET_CHECK failure (xls/dslx/bytecode/bytecode_emitter.cc:1088) proc_node != nullptr
0x5588a02eb970: xabsl::StatusBuilder::CreateStatusAndConditionallyLog()
0x55889ea06909: absl::lts_20220623::StatusOr<>::StatusOr<>()
0x55889e9f7ae6: xls::dslx::(anonymous namespace)::CreateChannelData()
0x55889e9f6c23: xls::dslx::BytecodeEmitter::HandleRecv()
0x55889ea62c2b: xls::dslx::Recv::AcceptExpr()
0x55889e9f4757: xls::dslx::BytecodeEmitter::HandleLet()
0x55889ea6364b: xls::dslx::Let::AcceptExpr()
0x55889e9e7c3a: xls::dslx::BytecodeEmitter::HandleBlock()
0x55889ea5fd38: xls::dslx::Block::AcceptExpr()
0x55889e9e4b21: xls::dslx::BytecodeEmitter::EmitProcNext()
0x55889e9e4954: xls::dslx::BytecodeEmitter::Emit()
0x55889e8b30f4: xls::dslx::BytecodeCache::GetOrCreateBytecodeFunction()
0x55889e9ca5db: xls::dslx::BytecodeInterpreter::GetBytecodeFn()
0x55889e9bab62: xls::dslx::BytecodeInterpreter::EvalCall()
0x55889e9b9d04: xls::dslx::BytecodeInterpreter::EvalNextInstruction()
0x55889e9b8975: xls::dslx::BytecodeInterpreter::Run()
0x55889e9dca2f: xls::dslx::ProcInstance::Run()
0x55889e84835f: xls::dslx::ParseAndTest()
0x55889e7fc068: main
0x7f03e4869083: __libc_start_main

E0502 10:10:52.469560 31692 run_routines.cc:379] Internal error: INTERNAL: XLS_RET_CHECK failure (xls/dslx/bytecode/bytecode_emitter.cc:1088) proc_node != nullptr
[ FAILED ] counter_test: internal error: INTERNAL: XLS_RET_CHECK failure (xls/dslx/bytecode/bytecode_emitter.cc:1088) proc_node != nullptr
[===============] 1 test(s) ran; 1 failed; 0 skipped.
```