[llvm] [AArch64] Enable fixed-length vector support for partial-reductions (PR #142032)

[Nicholas Guy via llvm-commits]

https://github.com/NickGuy-Arm approved this pull request.

Looks good to me. Though you will need to rebase it on top of https://github.com/llvm/llvm-project/commit/1651aa294342b2c019d3691fbfe890ee65194e1b

https://github.com/llvm/llvm-project/pull/142032