This PR improves the performance of and flattening in `bv_decide`.
The two main insights of this PR are:
1. When embedded constraint substitution is disabled it makes no sense
to have and flattening on in
the first place, given that we do not profit from it in any way.
2. The new fvars produced by and flattening can also be inserted into
the rewriting caches of the
preprocessing pipeline if the fvar they were derived from is already in
the cache. This
drastically decreases the amount of work we have to do in the second
rewriting pass after running
and flattening.
88f17dee71