[NFC] Demoted variable to function scope.
[NFC] Demoted variable to function scope.

The blocksWithReachedTops variable is only ever used while finding
barriers and in a small helper function called at that time.  Make it
local to that function and make that helper a lambda in that function.

roambotics/swift,benlangmuir/swift,rudkx/swift,atrick/swift,JGiola/swift,JGiola/swift,apple/swift,benlangmuir/swift,gregomni/swift,roambotics/swift,benlangmuir/swift,roambotics/swift,rudkx/swift,JGiola/swift,roambotics/swift,atrick/swift,ahoppen/swift,JGiola/swift,ahoppen/swift,apple/swift,gregomni/swift,apple/swift,roambotics/swift,gregomni/swift,glessard/swift,glessard/swift,glessard/swift,apple/swift,glessard/swift,roambotics/swift,ahoppen/swift,atrick/swift,atrick/swift,apple/swift,rudkx/swift,gregomni/swift,rudkx/swift,gregomni/swift,atrick/swift,glessard/swift,atrick/swift,benlangmuir/swift,benlangmuir/swift,ahoppen/swift,rudkx/swift,ahoppen/swift,rudkx/swift,ahoppen/swift,apple/swift,gregomni/swift,JGiola/swift,JGiola/swift,benlangmuir/swift,glessard/swift