Fix cat of zero-width SInt (bp #2116) (#2118)

* Fix cat of zero-width SInt (#2116)

Previously, concatenating two SInts where one is of zero-width would
return the non-zero-width SInt. This is incorrect because the output of
Cat should be of type UInt. Now the ZeroWidth transform will introduce a
cast when removing a Cat when the argument type is non-UInt.

(cherry picked from commit fd55c51b

)

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

* Fix merge conflicts

Co-authored-by: Jack Koenig <koenig@sifive.com>