proc_arst: Add special-casing of clock signal in conditionals.

proc_arst: Add special-casing of clock signal in conditionals.

The already-existing special case for conditionals on clock has been
remade as follows:

- now triggered for the last remaining edge trigger after all others
  have been converted to async reset, not just when there is only one
  sync rule in the first place
- does not require all contained assignments to be constant, as opposed
  to a reset conditional — merely const-folds the condition

In addition, the code has been refactored a bit; as a bonus, the
priority order of async resets found is now preserved in resulting sync
rule ordering (though this is not yet respected by proc_dff).

Fixes #2656.