Unprofitable loop vectorization

[nikic]

LoopVectorize generates something really bad here, with lots of element-wise loads, inserts, extracts and stores. Letting it fall through to SLP generates much better code.

LoopVectorize: https://llvm.godbolt.org/z/dPcEedY1v
SLPVectorizer: https://llvm.godbolt.org/z/33d31djno

[fhahn]

I think this should be fixed by #106441 or by a slightly generalization of it.

[fhahn]

(#106441 should land soon)

[artagnon]

I don't think this is fixed by #106441.

[nikic]

@fhahn As #106441 did not fix this, any idea on what more would be needed here?