[llvm] [SLP]Vectorize gathered loads (PR #107461)

[Mikael Holmén via llvm-commits]

mikaelholmen wrote:

> For info, I've bisected (what I think is) a miscompile to this patch. I'm trying to extract some kind of reproducer.

Reproducer:
```
clang bbi-100697.c -O2 -std=c23
./a.out
```
After 1833d418a04 this results in
```
Fail!
```
and before it results in
```
Pass!
```
I have no idea what goes wrong, if SLP does anything bad or if it just triggers a bug in some later pass.
The input was originally part of a much larger test so I've reduced it a lot but unfortunately it's still quite large (and ugly).

[bbi-100697.c.gz](https://github.com/user-attachments/files/17572329/bbi-100697.c.gz)


https://github.com/llvm/llvm-project/pull/107461