[llvm] [VPlan] Add transformation to narrow interleave groups. (PR #106441)

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

This change caused a miscompile in ffmpeg, on aarch64 (on both Linux and Windows), causing ffmpeg to crash. The issue is reproducible both with ffmpeg compiled via llvm-test-suite, and compiled standalone (clone https://github.com/ffmpeg/ffmpeg, `./configure --cc=clang && make -j$(nproc)`).

To reproduce the crash, build ffmpeg in either way and run:
```
$ ./External/ffmpeg/ffmpeg -s 352x288 -i http://martin.st/temp/vsynth1.yuv -c ffv1 -coder range_tab -pass 1 -y vsynth1-ffv1-2pass.avi
```
(This uses an input file hosted by me, for repeated use it may be easier to download it and reference it with a local file name.)

The miscompiled source file is `libavcodec/ffv1enc.c`.

https://github.com/llvm/llvm-project/pull/106441