v68k:  Slightly optimize decode_MOVE()

It's more efficient to compare against zero than one.
v68k:  Don't "storage.flags |=" in decode_line_0()
v68k:  Refactor decode_ADD_SUB() for efficiency
Merge branch 'optimize-v68k-decoder' into optimize-v68k
Merge branch 'optimize-v68k'
v68k:  Make decoder_per_line[] members const
v68k:  Axe unused has_0100 in decode_line_B()