This extends optimizeCompareInstr to re-use previous comparison results if the previous comparison was with an immediate that was 1 bigger or smaller. Example:
CMP x, 13 ... CMP x, 12 ; can be removed if we change the SETg SETg ... ; x > 12 changed to `SETge` (x >= 13) removing CMP
Motivation: This often happens because SelectionDAG canonicalization tends to add/subtract 1 often when optimizing for fallthrough blocks. Example for x > C the fallthrough optimization switches true/false blocks with !(x > C) --> x <= C and canonicalization turns this into x < C + 1.