Skip ghost instructions when coloring.
Skip ghost instructions when coloring.

Ghost instructions don't have an encoding, and don't appear in the
output. The values they define do not need to be assigned to registers,
so they can be skipped.

sunfishcode/cretonne,sunfishcode/cretonne,stoklund/cretonne,sunfishcode/cretonne,stoklund/cretonne,stoklund/cretonne