[llvm] Reland "[LoopVectorizer] Add support for chaining partial reductions #120272" (PR #124282)

[Sam Tebbs via llvm-commits]

https://github.com/SamTebbs33 approved this pull request.

LGTM. I like the new `getScaledReductions` interface.

https://github.com/llvm/llvm-project/pull/124282