[llvm] [SLP]Improve minbitwidth analysis for trun'ed gather nodes. (PR #99072)

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

I bisected a miscompilation (giving the wrong result at runtime) back to this commit, reproducing on x86_64 Linux.

To repro:
```
$ git clone https://github.com/ffmpeg/ffmpeg
$ cd ffmpeg
$ git checkout n6.0
$ mkdir ../ffmpeg-build
$ cd ../ffmpeg-build
$ ../ffmpeg/configure --cc=clang --samples=$(pwd)/../samples
$ make fate-rsync
$ make -j$(nproc) fate-copy-trac4914-avi
```

Note that the issue only appears in an older version of ffmpeg, it doesn’t seem to happen with latest git. The miscompilation seems to happen in the `libavformat/avformat.o` object file.

https://github.com/llvm/llvm-project/pull/99072