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

[Nicholas Guy via llvm-commits]

NickGuy-Arm wrote:

> LGTM with suggestion inline and please double-check the tests pass with ASan/UBSan

Thanks. I have tested with both sanitizers, and the relevant tests no longer fail with them. However I am seeing some other test failures, but those fail even without these changes (and one seemingly flakey test that comes and goes with or without this PR). Regardless, I'll wait until tomorrow to land this patch just in case it does cause things to fail again.

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