[Transforms/ObjCARC] Fix non-deterministic output of `ObjCARCOptPass`

`ProvenanceAnalysis::related()` was assuming that the order of parameters for `relatedCheck()` was not affecting
the result but this was not the case when both parameters were `PHINode`s.
Due to this assumption `ProvenanceAnalysis::related()` was ordering the parameters based on pointer value which resulted in
non-deterministic behavior.

To address this change `relatedPHI()` so that it gives the same result independent of the parameter order.

rdar://100325456

Differential Revision: https://reviews.llvm.org/D135376