[llvm] Add LoopVectorizer support for `llvm.vector.partial.reduce.fadd` (PR #163975)

[Sam Tebbs via llvm-commits]

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+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 6
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SamTebbs33 wrote:

I think we'll want some replicated versions of the `not_*` tests in partial-reduce-dot-product.ll just to make sure the same cases are caught for floating point partial reductions.

https://github.com/llvm/llvm-project/pull/163975