These 2 routines were adding and subtracting 1 to a range bound, and 
checking for overflow.

Signed 1 bit values have a range of [-1, 0]. Adding or subtracting 1 
cannot be properly represented resulting in the overflow being set.  
This caused us to set UNDEFINED when we shouldn't.

This patch changes it such that signed values always have -1 added or 
subtracted as appropriate, allowing for proper representation and 
overflow setting.

Bootstrapped on x86_64-pc-linux-gnu with no regressions.  Pushed.

Andrew