[dv] Formal flow dependency

[marnovandermaas]

We should encode the patch for yosys-slang into lowrisc-nix: #2309 (comment)

[nbdd0121]

yosys-slang maintainer expresses interest in taking the change upstream: #2309 (comment)

[hcallahan-lowrisc]

yosys-slang maintainer expresses interest in taking the change upstream: #2309 (comment)

@mndstrmr Are the changes suitable for proposing to upstream as-is? Or is there anything you would like to change / cleanup before doing so?