opt_mem_priority - remove priority relations between write ports that can never collide

    opt_mem_priority [selection]

This pass detects cases where one memory write port has priority over another
even though they can never collide with each other -- ie. there can never be
a situation where a given memory bit is written by both ports at the same
time, for example because of always-different addresses, or mutually exclusive
enable signals. In such cases, the priority relation is removed.