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

[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@231439 91177308-0d34-0410-b5e6-96231b3b80d8