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

[Sam Tebbs via llvm-commits]

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+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 4
+; RUN: opt -S -o - %s --passes=loop-vectorize -force-vector-width=16 -force-vector-interleave=1 -scalable-vectorization=on | FileCheck %s
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SamTebbs33 wrote:

Is `-scalable-vectorisation` required if we have SVE enabled? I think it would be better to add these tests to the existing partial reduction test files, unless there's a reason why they can't share run lines. If there is a reason for that then we'll want a file for neon and a file for mixed extend types.

https://github.com/llvm/llvm-project/pull/120272