> minority use-case

The amount of code that looks like this in a big enough hot loop to make a difference is negligible. Can you provide even one real-world example where this makes a difference, i.e. not some microbenchmark? The amount of code that can break as a result of signed overflows being UB, on the other hand, is huge.

> programmers would have to do a lot more fragile hand-unrolling of operations to get that performance back

Much easier ways to do this, e.g. by using an assert wrapper around __builtin_unreachable. Alternatively, an unsafe_int_t could be defined that gives the optimize-able behavior. The important thing is to make it opt-in; sensible defaults matter.