tnum_with_subreg
struct tnum tnum_with_subreg(struct tnum reg, struct tnum subreg);
return tnum_with_subreg(a, tnum_const(value));
reg1->var_off = tnum_with_subreg(reg1->var_off, t);
reg2->var_off = tnum_with_subreg(reg2->var_off, t);
reg1->var_off = tnum_with_subreg(reg1->var_off, t);
reg1->var_off = tnum_with_subreg(reg1->var_off, t);