[llvm] [ARM][SLP] Fix cost function for SLP Vectorization of ZExt/SExt (PR #122713)

[Nashe Mncube via llvm-commits]

nasherm wrote:

> You just need to modify TTI::getExtendedReductionCost function, no changes in SLP are required. Also, provide a test

I've updated the patch with this in mind

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