[llvm] [RISCV][CostModel] Refine Arithmetic reduction costs (PR #79103)

[Luke Lau via llvm-commits]

https://github.com/lukel97 approved this pull request.

LGTM. Assuming that the spec tests were built with -O2/-O3 it makes sense that there's not any changes, seeing as most of the diffs here are for the size cost.

https://github.com/llvm/llvm-project/pull/79103