Use the actual written address range in DC ZVA's memory operand

Unlike the other DC cache operations, DC ZVA is not a cache management
operation but more of a regular store instruction. But clients analyzing
it need its actual start address (back-aligned) and size (cache line size).
The easiest thing for a client would be to have that in the IR operand,
instead of the encoded address: though that brings up issues of re-encoding
fidelity (xref #3339).