reorganise src and dest vector-element offsets

reorganise src and dest vector-element offsets

pass in pointer to offsets from processor_t->state.srcoffs and destoffs
into sv_insn_t so that the element state information about how many
parallel elements have been executed is recorded.

in this way it will be possible to restart a loop at the right place
if there is an exception (such as a memory cache miss on a LOAD/STORE)