[llvm] [CodeGen] Add ValueType v3i8 (NFCI). (PR #80826)

[Francesco Petrogalli via llvm-commits]

fpetrogalli wrote:

Hang on, genuine failures in https://buildkite.com/llvm-project/github-pull-requests/builds/36346#018d7e21-6650-4dd6-b687-9f472dbd9145


```
  LLVM :: CodeGen/AArch64/vec3-loads-ext-trunc-stores.ll
```

https://github.com/llvm/llvm-project/pull/80826