enable the mergefunc pass #9536
Description
Replacing #9222 with this bug to stop tying it to the removal of type_use.
This pass is almost robust enough, but still hits an assert on libstd. It does actually work on libextra already. There is work happening upstream on it right now, and one of the known issues is inability to create aggregate casts correctly (which is likely the CastInst assert this hits).