Using assert fmt with reset fails on functions

[grebe]

Describe the bug
Using an assert fmt string can fail when codegening something that doesn't generate registers. Even if you specify a reset, the current behavior means it won't be populated unless there's a user. The way op overrides add an assert format string means that it won't look like a reset user at block conversion. You end up with an error like: Assert format string has {rst} placeholder, but block has no reset signal..

To Reproduce
Steps to reproduce the behavior:

1. Make a function with an assertion that will codegen with no registers (e.g. set pipeline_stages=1)
2. Run codegen with an assert format string that uses {rst}.
3. See error

Expected behavior
This should codegen properly with a reset port that gets sent to the assert fmt string.