feat[venom]: improve memmerge pass (#4422)

this commit improves the memmerging pass by only flushing invalidated
regions where possible, instead of always flushing everything on an
invalidation.

it refactors the memmerge pass, including splitting out the different
kinds of invalidations for clarity.

---------

Co-authored-by: Charles Cooper <[email protected]>

Author: HodanPlodky