> How it works exactly I don't know

The idea is pretty simple. You can build a list of known facts based on control flow, explicit __builtin_assumes, and undefined behavior relations. For example, if you've got this code:

  if (x < N) {
    // In this block, we know that x < N
  } else {
    // ... and in this block we know that x >= N!
  }

And on top of that, we can do some basic algebra. If we know that x < N and N < 5, then we can infer that x < 5. So if we see a comparison x < 5, we can then rewrite that to true.

> and apparently it's so complex that it requires over 9000 lines of C++ to express

The two main reasons for that is that a) there is a lot of rules covering cases like "we know the result of count_leading_zeroes can be no more than the number of bits in an integer" and so forth, and b) this is doing a lot more logic than just tracking integer comparisons: there's tracking known-bits of integers, maximum possible value, floating-point comparisons, pointer object references.