[ObjC][ARC] In HandlePotentialAlterRefCount, check whether an

instruction can decrement the reference count, not whether it can alter
it

This prevents the state transition from S_Use to S_CanRelease when doing
a bottom-up traversal and the transition from S_Retain to S_CanRelease
when doing a top-down traversal when the visited instruction can
increment the ref count but cannot decrement it. This allows the ARC
optimizer to remove retain/release pairs which were previously not
removed.

rdar://problem/21793154