[llvm] [SelectionDAG] Use Karatsuba decomposition to expand vector CLMUL via narrower legal types (PR #184468)

[Jay Foad via llvm-commits]

jayfoad wrote:

I don't think "Karatsuba" is a good name for this, unless there is some precedent for it? For regular multiplication the Karatsuba algorithm specifically involves doing only _three_ half-width multiplications in the divide and conquer step.

https://github.com/llvm/llvm-project/pull/184468