optimised INCR & DECR variable

[boriel]

Because I also coded some stuff in AMOS on Amiga: How about optimised INCR and DECR commands for variables? This can be easyly optimised because Z80 knows INC and DEC commands, and variable=variable+1 is maybe much slower.
INCR variable increases the value by one. This will be very handy, I think .

Well, using -O3 should convert variable=variable+1 to inc(variable). If not, its something to be done.