[objc-arc] Move the checking of whether or not we can match onto PtrStates and...

[objc-arc] Move the checking of whether or not we can match onto PtrStates and out of the main dataflow.

These refactored computations check whether or not we are at a stage
of the sequence where we can perform a match. This patch moves the
computation out of the main dataflow and into
{BottomUp,TopDown}PtrState.

llvm-svn: 231439