Fix bug in zero-width memory removal (bp #2153) (#2155)

* Fix bug in zero-width memory removal (#2153)

* Fix bug in zero-width memory removal

Correctly remove all extraneous connections to all types of memory
ports (read, write, readwrite) for zero-width memories.  Previously,
only read ports were correctly handled.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>

* fixup! Fix bug in zero-width memory removal

(cherry picked from commit 67ce97a1

)

# Conflicts:
#	src/main/scala/firrtl/passes/ZeroWidth.scala

* fixup! Fix bug in zero-width memory removal (#2153)

Co-authored-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>