RTS memory return behaviour is incorrect for non-moving GC
In !10381 (comment 498455): @teo says:
@mpickering apologies for not catching this before this got merged, but I don't think this is right for the case when
gen
isoldest_gen
and we are using the non-moving gc. Thenoldest_gen->n_blocks
should be counted along with uncopied things as it's the blocks that make up segments I think