[ISel] Improve `clmul` fallback implementation (#204802)
Generalize the approach from
https://github.com/llvm/llvm-project/pull/203727 to narrower and wider
integers.
We still need the fallback for when multiplication isn't available, and
it turns out that for some widths the fallback emits fewer instructions,
the naive fallback is still used for `i1`, `i3`, `i4` and `i9`. I've
also now enabled wider integers (`i128` and `i256` have uses in
cryptography).
Based on my local experiments, the Karatsuba approach (e.g. as in
https://github.com/rust-lang/rust/pull/152132#discussion_r2778609222) is
not actually better than zero extending the input and using
multiplication with holes on the wider type.
CC https://github.com/llvm/llvm-project/issues/203694
CC: @eisenwave
