[llvm] Optimize circular buffers (PR #75722)

[Felipe de Azevedo Piovezan via llvm-commits]

felipepiovezan wrote:

I don't think the compiler performs the optimization is either case:
https://godbolt.org/z/xqeY8zaE6

FWIW unsigned overflow is defined behavior, so `x +1 == end` will do the _wrong_ thing if `x` is unsigned and `x+1` overflows and the type of `end` is something larger like uint64_t. 

The same situation when `x` is `int` is undefined behavior and would be caught by tooling.

https://github.com/llvm/llvm-project/pull/75722