[DSLX/IR/Codegen] Inlining IR with inter proc channels fails, fifo_depth must be < 1 but was (none)

I've encountered an issue with a design that uses 2 procs.
The first proc receives data from external channel, does some processing and
it sends data to the second proc. Second proc also does some processing and
it sends its results to an external channel.

During code generation I get error:
```
Only streaming channels of FIFO depth one or less are supported in proc inlining. Channel `proc1_proc2` has depth: (none)
```
I've modified DLSX to IR converter, so that every intra DLSX channel has depth of 1.
It was enough to get codegen working but as i can see in #950 multiproc codegen is being developed.
I think this error is similar to #708.

I'll create PR with my current fix.
/cc @proppy

PR: https://github.com/google/xls/pull/977