mem: Avoid adding and then removing empty snoop-filter items
mem: Avoid adding and then removing empty snoop-filter items

This patch tidies up how we access the snoop filter for snoops, and
avoids adding items only to later remove them.

Weil0ng/gem5,qizenguf/MLC-STT,gem5/gem5,TUD-OS/gem5-dtu,TUD-OS/gem5-dtu,aclifton/cpeg853-gem5,sobercoder/gem5,Weil0ng/gem5,cancro7/gem5,powerjg/gem5-ci-test,briancoutinho0905/2dsampling,briancoutinho0905/2dsampling,powerjg/gem5-ci-test,gem5/gem5,cancro7/gem5,rjschof/gem5,rjschof/gem5,cancro7/gem5,KuroeKurose/gem5,SanchayanMaity/gem5,KuroeKurose/gem5,gedare/gem5,TUD-OS/gem5-dtu,rallylee/gem5,powerjg/gem5-ci-test,sobercoder/gem5,SanchayanMaity/gem5,HwisooSo/gemV-update,TUD-OS/gem5-dtu,qizenguf/MLC-STT,HwisooSo/gemV-update,rjschof/gem5,powerjg/gem5-ci-test,cancro7/gem5,SanchayanMaity/gem5,KuroeKurose/gem5,powerjg/gem5-ci-test,HwisooSo/gemV-update,Weil0ng/gem5,rallylee/gem5,gedare/gem5,gem5/gem5,sobercoder/gem5,rjschof/gem5,qizenguf/MLC-STT,gedare/gem5,powerjg/gem5-ci-test,sobercoder/gem5,TUD-OS/gem5-dtu,aclifton/cpeg853-gem5,Weil0ng/gem5,rallylee/gem5,aclifton/cpeg853-gem5,qizenguf/MLC-STT,qizenguf/MLC-STT,aclifton/cpeg853-gem5,powerjg/gem5-ci-test,rjschof/gem5,gedare/gem5,gedare/gem5,qizenguf/MLC-STT,KuroeKurose/gem5,gem5/gem5,aclifton/cpeg853-gem5,sobercoder/gem5,gem5/gem5,cancro7/gem5,briancoutinho0905/2dsampling,Weil0ng/gem5,rjschof/gem5,briancoutinho0905/2dsampling,gem5/gem5,gedare/gem5,sobercoder/gem5,gem5/gem5,SanchayanMaity/gem5,briancoutinho0905/2dsampling,HwisooSo/gemV-update,qizenguf/MLC-STT,rallylee/gem5,sobercoder/gem5,TUD-OS/gem5-dtu,TUD-OS/gem5-dtu,briancoutinho0905/2dsampling,Weil0ng/gem5,cancro7/gem5,HwisooSo/gemV-update,rallylee/gem5,KuroeKurose/gem5,rallylee/gem5,HwisooSo/gemV-update,KuroeKurose/gem5,KuroeKurose/gem5,HwisooSo/gemV-update,aclifton/cpeg853-gem5,cancro7/gem5,briancoutinho0905/2dsampling,rjschof/gem5,SanchayanMaity/gem5,SanchayanMaity/gem5,aclifton/cpeg853-gem5,rallylee/gem5,Weil0ng/gem5,SanchayanMaity/gem5,gedare/gem5