optimizing for loops
What you read is total nonsense, except for the most primitive of compilers. First, comparison with an integer is as fast, possibly even faster than comparison with a constant. Second, a good optimising compiler will take a loop written using some common pattern and turn it into the best possible code; it may not recognise your obfuscated pattern and produce less good code for it.
And lastly, you shouldn’t replace readable with unreadable code unless there is an actual need for it. If you spend an hour making the change, it needs to produce 20 hours saved CPU time at least. When you are at that level, better algorithms are very likely to give better savings.