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

[Francesco Petrogalli via llvm-commits]

fpetrogalli wrote:

The current version will fail these two AMDGPU CodeGen tests: 

```
  LLVM :: CodeGen/AMDGPU/calling-conventions.ll
  LLVM :: CodeGen/AMDGPU/store-v3i64.ll
```

@arsenm, are you able to look into this? I blamed places in the AMDGPU backend that mention truncating stores to i8, you seem to have workedd with them. Can you help out here? My knowledge of this ISA is close to zero...

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