That said, I think it's less of an issue these days for JS implementors in particular. It might have mattered more back in the day when pure JS carried a lot of numeric compute load and there weren't other options. These days it's better to stow that compute code in wasm and get predictable reliable performance and move on.
The big pain points in perf optimization for JS is objects and their representation, functions and their various type-specializations.
Another factor is that JS impls use int32s as their internal integer representation, so there should be some relatively straightforward approach involving lifting to int64s and testing the high half for overflow.
Still kind of cumbersome.
There are similar issues in existing ISAs. NaN-boxing for example uses high bits to store type info for boxed values. Unboxing boxed values on amd64 involves loading an 8-byte constant into a free register and then using that to mask out the type. The register usage is mandatory because you can't use 64-bit values as immediates.
I remember trying to reduce code size and improve perf (and save a scratch register) by turning that into a left-shift right-shift sequence involving no constants, but that led to the code executing measurably slower as it introduced data dependencies.
