[flow-isolation] Track property uses via Operand * instead of SILInstruction *
Change propertyUses from SmallPtrSet<SILInstruction *, 8> to
SmallPtrSet<Operand *, 8> so that each recorded use captures both the
violating instruction and the specific operand value. This enables use
of the SIL naming infrastructure to produce better diagnostics.
Callers of markPropertyUse() are updated to pass the operand rather than
the instruction, and sites that previously called ->getParent() on a
SILInstruction now call ->getParentBlock() on the Operand.