Reduce the complexity of function Lower.

According to Clang-Tidy's [readability-function-cognitive-complexity](https://clang.llvm.org/extra/clang-tidy/checks/readability/function-cognitive-complexity.html), the recommended threshold is 15. However, the function `Lower` had a cognitive complexity of 134. This number is now reduced to 18. Although it's still higher than the threshold, it's in a much better state than before.

PiperOrigin-RevId: 676086424