[enhancement] Specify I/O options per-channel

[hongted]

What's hard to do? (limit 100 words)

Currently I/O options for channels are global and specified as side-band options. https://google.github.io/xls/codegen_options

This is too generic and often we would like to specify the options differently (ex. loop-back channels should not be flopped) while X output channel should have a fifo depth of 5, and Z output channel should have a fifo depth of 4.

Current best alternative workaround (limit 100 words)

Right now, the option is to have no I/O options and insert the additional options manually in Verilog.

Your view of the "best case XLS enhancement" (limit 100 words)

Two options are considered

1. Enhance DSLX to support tags/configurations on each channel. (ex. this channel should have fifo depth of 5, or this channel should have no flops added).
2. Enhance codegen options to specify options per-channel.