arch: get rid of dummy var init

arch: get rid of dummy var init

MemOperand variables were being initialized to 0
"to avoid 'uninitialized variable' errors" but these
no longer seem to be a problem (with the exception of
one use case in POWER that is arguably broken and
easily fixed here).

Getting rid of the initialization is necessary to
set up a subsequent patch which extends memory
operands to possibly not be scalars, making the
'= 0' initialization no longer feasible.