Auto merge of #144393 - heiher:str-contains-lsx, r=tgross35

LoongArch64 LSX fast-path for `str.contains(&str)`

Benchmark results with LLVM 21 on LA664:

```
OLD:
test bench_is_contained_in ... bench:          43.63 ns/iter (+/- 0.04)

NEW:
test bench_is_contained_in ... bench:          12.81 ns/iter (+/- 0.01)
```

Author: bors