[llvm] MathExtras: rewrite some methods to never overflow (PR #95556)

[Nikita Popov via llvm-commits]

nikic wrote:

It looks like this does have some overhead in practice: https://github.com/llvm/llvm-project/commit/bfd95a003139a8f930874b2234c3cab545d504a1

It looks like LLVM currently can't optimize this even if overflow isn't possible: https://github.com/llvm/llvm-project/issues/95652

https://github.com/llvm/llvm-project/pull/95556