def-use: fix computing affected memory size
def-use: fix computing affected memory size

If the instruction given is a store inst, we need to use
the value operand, not the store itself, because it is not
sized

Signed-off-by: Marek Chalupa <a05df156a69ad59faa9af9b666ef78009594c145@gmail.com>

mchalupa/dg,mchalupa/dg,mchalupa/dg,mchalupa/dg