[IR] Remove MSVC warning workaround (NFC)

While LLVM does fold this to x+1, GCC does not. As this is hot
code, let's try to avoid that.

According to
https://developercommunity.visualstudio.com/content/problem/211134/unsigned-integer-overflows-in-constexpr-functionsa.html
this spurious warning in MSVC has been fixed in Visual Studio 2019
Version 16.4. Let's see if there are any build bots running old
MSVC versions with warnings treated as errors...