possible input transformations
	adde	$-1,X	=> addme X
	adde	$0,X	=> addze X
	subw	$s,X	=> addw	$-s,X
	orn	$v,X	=> or	$~v,X

qa:
	subc	r1,$s,r2 => subc $s,r1,r2
	movw	sreg(Rn),Rm => movw sreg(NREG),Rn,Rm  [and v.v.]

others?
	andn	$m => and	$~m
	slw	$sh,s,a => rliwnm
	srw	$sh,s,a => rliwnm

support for C_LCON needed since addresses are literals?

- moves
- branch distance

- could rewrite movwu x,d(r) as movw $d,tmp; movwu x,(tmp+d) when d is large?