[enhancement] IR Conversion could have a default FIFO config

[grebe]

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

Currently, the ways to specify FIFO configuration are limited and tedious (see #1561 as wells as #1391 + #989). Furthermore, because FIFO configurations are only relevant at codegen, you can write a bunch of DSLX, test it, and then have to go back and update every chandecl to get it to codegen.

Current best alternative workaround (limit 100 words)

Manually specify DEPTH everywhere until things build.

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

In lieu of enhancements in #1561 making FIFO specification easier and better, it is easy to implement and helpful to have an IR conversion option where you specify a default FIFO configuration. It could be something like --default_fifo_config=<textproto of FifoConfig>