[llvm] [SLP]Improved reduction cost/codegen (PR #118293)

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

This change miscompiles one file in ffmpeg, for x86 and x86_64.

To reproduce, clone https://github.com/ffmpeg/ffmpeg, compile and run tests like this:
```
$ git clone https://github.com/ffmpeg/ffmpeg
$ mkdir ffmpeg-build
$ cd ffmpeg-build
$ ../ffmpeg/configure --cc=clang --samples=$(pwd)/../ffmpeg-samples
$ make fate-rsync
$ make -j$(nproc) fate-msnsiren
```

The miscompiled object file is `libavcodec/siren.o`.

https://github.com/llvm/llvm-project/pull/118293