This commit fixes the implementation of 'regUsageOfInstr' for vector
operations that take an 'Operand' as the destination, by ensuring that
when the destination is an address then the address should be *READ*,
and not *WRITTEN*.

Getting this wrong is a disaster, as it means the register allocator
has incorrect information, which can lead to it discard stores to
registers, segfaults ensuing.

Fixes #25486