[llvm] use multiplication with holes for 32-bit/64-bit clmul fallback (PR #203727)

[Jan Schultke via llvm-commits]

eisenwave wrote:

Do you know how this performs compared to the suggested approach outlined in https://github.com/llvm/llvm-project/issues/203694? You can find some quick-bench there and an implementation that works for a `_BitInt` of any size, so it shouldn't be too hard to compare.

I'm mainly concerned about having too many different implementations, especially if what you're adding only covers between 32 and 64 bits. That still doesn't get rid of the worst-case linear implementation.

https://github.com/llvm/llvm-project/pull/203727