ScopInfo: Do not modify the original alias group [NFC]

Instead of modifying the original alias group and repurposing it as read-write
access group when splitting accesses in read-only and read-write accesses, we
just keep all three groups: the original alias group, the set of read-only
accesses and the set of read-write accesses.  This allows us to remove some
complicated iterator handling and also allows for more code-reuse in
calculateMinMaxAccess.

llvm-svn: 292137