[llvm] [AArch64][NFC] Move getPartialReductionCost into cpp file (PR #123370)

[David Green via llvm-commits]

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

Perhaps put the code close to getArithmeticReductionCost, as they are conceptually similar?

Either way LGTM too.

https://github.com/llvm/llvm-project/pull/123370