[llvm] Add `llvm.vector.partial.reduce.fadd` intrinsic (PR #159776)

[Paul Walker via llvm-commits]

paulwalker-arm wrote:

You'll need to roll out the `reassoc` change to the LangRef as well, so the unordered nature is activated by this option but then add a separate note stating the flag is required.

https://github.com/llvm/llvm-project/pull/159776