chan array should be indexable w/ values deduced from proc parameters

[proppy]

Describe the bug
It seems that one can't currently index array w/ values deduced from proc parameters.

To Reproduce

proc matmul<ROWS: u32, COLS: u32> {
    activations_in: chan<F32>[ROWS] in;
    results_out: chan<F32>[COLS] out;
    west_inputs: chan<F32>[COLS + u32:1][ROWS] in;
    east_outputs: chan<F32>[COLS + u32:1][ROWS] out;
    north_inputs: chan<F32>[COLS][ROWS + u32:1] in;
    south_outputs: chan<F32>[COLS][ROWS + u32:1] out;

    config(activations_in: chan<F32>[ROWS] in, results_out: chan<F32>[COLS] out) {
        let (east_outputs, west_inputs) = chan<F32>[COLS + u32:1][ROWS]("east_west");
        let (south_outputs, north_inputs) = chan<F32>[COLS][ROWS + u32:1]("north_south");
        unroll_for! (row, _): (u32, ()) in u32:0..ROWS {
            unroll_for! (col, _): (u32, ()) in u32:0..COLS {
                let weight = F32 {
                    sign: false,
                    bexp: if col == row { u8:128 } else { u8:0 },
                    fraction: u23:0,
                };
                spawn node(
                    west_inputs[row][col], north_inputs[row][col], east_outputs[row][col + u32:1],
                    south_outputs[row + u32:1][col], weight);
            }(());
        }(());
        (activations_in, results_out, west_inputs, east_outputs, north_inputs, south_outputs)
    }

    init { () }

    next(state: ()) {
        let activations_col = u32:0;
        unroll_for! (row, _): (u32, ()) in u32:0..ROWS {
            let (tok, activation) = recv(join(), activations_in[row]);
            send(tok, east_outputs[row][activations_col], activation);
        }(());
        let zeroes_row = u32:0;
        unroll_for! (col, _): (u32, ()) in u32:0..COLS {
            send(join(), south_outputs[zeroes_row][col], float32::zero(false));
        }(());
        let drops_col = COLS + u32:1;
        unroll_for! (row, _): (u32, ()) in u32:0..ROWS {
            recv(join(), west_inputs[row][drops_col]);
        }(());
        let results_row = ROWS + u32:1;
        unroll_for! (col, _): (u32, ()) in u32:0..COLS {
            let (tok, result) = recv(join(), north_inputs[results_row][col]);
            send(tok, results_out[col], result);
        }(());
    }
}

will produce the following error:

F1128 00:52:50.016040    4046 channel_scope.cc:417] Check failed: !subarray->flattened_names_in_order().empty() 
*** Check failure stack trace: ***
absl::log_internal::LogMessage::SendToLog()
absl::log_internal::LogMessage::Flush()
absl::log_internal::LogMessageFatal::~LogMessageFatal()
xls::dslx::ChannelScope::GetOrDefineSubarray()
xls::dslx::ChannelScope::GetChannelArrayElement()
xls::dslx::ChannelScope::EvaluateIndex()
xls::dslx::ChannelScope::GetChannelOrArrayForArrayIndex()
xls::dslx::FunctionConverter::HandleIndex()
xls::dslx::FunctionConverterVisitor::Visit()
xls::dslx::FunctionConverter::HandleBuiltinRecv()
xls::dslx::FunctionConverter::HandleInvocation()
xls::dslx::FunctionConverterVisitor::Visit()
std::__u::__variant_detail::__visitation::__base::__dispatcher<>::__dispatch<>()
xls::dslx::FunctionConverter::HandleStatement()
xls::dslx::FunctionConverterVisitor::Visit()
xls::dslx::FunctionConverter::HandleStatementBlock()
xls::dslx::FunctionConverterVisitor::Visit()
std::__u::__variant_detail::__visitation::__base::__dispatcher<>::__dispatch<>()
xls::dslx::FunctionConverter::HandleStatement()
xls::dslx::FunctionConverterVisitor::Visit()
xls::dslx::FunctionConverter::HandleStatementBlock()
xls::dslx::FunctionConverterVisitor::HandleUnrollFor()
xls::dslx::FunctionConverterVisitor::Visit()
std::__u::__variant_detail::__visitation::__base::__dispatcher<>::__dispatch<>()
xls::dslx::FunctionConverter::HandleStatement()
xls::dslx::FunctionConverterVisitor::Visit()
xls::dslx::FunctionConverter::HandleStatementBlock()
xls::dslx::FunctionConverterVisitor::Visit()
xls::dslx::FunctionConverter::HandleProcNextFunction()
xls::dslx::(anonymous namespace)::ConvertCallGraph()
xls::dslx::ConvertModuleIntoPackage()
xls::dslx::ConvertModuleToPackage()
xls::dslx::AbstractTestRunner::ParseAndTest()
main
__libc_start_main
_start

Expected behavior
chan array can be indexes from arbitrary computed values.

[proppy]

Adding a dummy outer unroll_for to capture the constant seems to workaround the issue:

        unroll_for! (row, _): (u32, ()) in ROWS..ROWS + u32:1 {
            unroll_for! (col, _): (u32, ()) in u32:0..COLS {
                let (tok, result) = recv(join(), north_inputs[row][col]);
                send(tok, results_out[col], result);
            }(());
        }(());