Optimize isBlockAliasedFromMasterCollection

- Fix PHPDoc: $b is a Block instance, not an array
- Fix argument: declaring that $b is passed by reference is useless (so let's use $b instead of &$b)
- If $b is not an alias let's return false without filling blocksAliasedFromMasterCollection (that's useless)
- $db->execute is useless, since it's already called inside $db->GetCol