mem: Avoid unnecessary snoops on writebacks and clean evictions
mem: Avoid unnecessary snoops on writebacks and clean evictions

This patch optimises the handling of writebacks and clean evictions
when using a snoop filter. Instead of snooping into the caches to
determine if the block is cached or not, simply set the status based
on the snoop-filter result.

pombredanne/http-repo.gem5.org-gem5-,pombredanne/http-repo.gem5.org-gem5-,pombredanne/http-repo.gem5.org-gem5-,pombredanne/http-repo.gem5.org-gem5-,pombredanne/http-repo.gem5.org-gem5-,pombredanne/http-repo.gem5.org-gem5-,pombredanne/http-repo.gem5.org-gem5-