[llvm] [ISel] Improve `clmul` fallback implementation (PR #204802)

[Folkert de Vries via llvm-commits]

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folkertdev wrote:

Here the new approach is apparently worse in terms of instruction count. Based on some grepping a big difference is the number of spills. Anything we can/should do about that?

https://github.com/llvm/llvm-project/pull/204802