[llvm] [AArch64][CostModel] Reduce the cost of fadd reduction with fast flag (PR #108791)

[David Green via llvm-commits]

https://github.com/davemgreen approved this pull request.

Thanks, from what I can tell the code here looks good. The comments I feel could do with a bit of an adjustment. If you can clean those up, then this LGTM.

https://github.com/llvm/llvm-project/pull/108791