sval: make cast_rl() preserve things better across signed casts

sval: make cast_rl() preserve things better across signed casts

In the old code if you had the condition:
	int x;

	if (x < (unsigned int)99) { ...

Then it would say that the true state is x == 0-5 and the false state is:
s32min-s32max.  That's obviously not correct.  What we want is the false
state to say x=s32min-(-1),99-s32max.

This patch does that.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>