Refactor common parts of SAT-using optimizations into a helper.

Refactor common parts of SAT-using optimizations into a helper.

This also aligns the functionality:

- in all cases, the onehot attribute is used to create appropriate
  constraints (previously, opt_dff didn't do it at all, and share
  created one-hot constraints based on $pmux presence alone, which
  is unsound)
- in all cases, shift and mul/div/pow cells are now skipped when
  importing the SAT problem (previously only memory_share did this)
  — this avoids creating clauses for hard cells that are unlikely
  to help with proving the UNSATness needed for optimization