opt_clean: Better memory handling.

opt_clean: Better memory handling.

Previously, `$memwr` and `$meminit` cells were always preserved (along
with the memory itself).  With this change, they are instead part of the
main cell mark-and-sweep pass: a memory (and its `$meminit` and `$memwr`
cells) is only preserved iff any associated `$memrd` cell needs to be
preserved.