[llvm] [ARM][SLP] Adjust cost returned for vadd reduction (PR #122713)

[Nashe Mncube via llvm-commits]

nasherm wrote:

I've opted for adjusting the cost returned by getExtendedReductionCost for vadd reductions to be slightly more pessimistic. This returns performance numbers on regressed benchmarks while not changing SLPVectorizer and making minimal changes to the rest of the compiler.

https://github.com/llvm/llvm-project/pull/122713