In order to fold an always overflowing signed saturating add/sub, we need to know in which direction the always overflow occurs. This patch splits up AlwaysOverflows into AlwaysOverflowsLow and AlwaysOverflowsHigh to pass through this information (but it is not used yet).
These now need a comment for each.
Yeah, this is the direction of overflow. For AlwaysOverflowLow the real result would be lower than signed/unsigned min, for AlwaysOverflowHigh it would be higher than signed/unsigned max.