[clang-tools-extra] [NVPTX] Improve lowering of v4i8 (PR #67866)

[Artem Belevich via cfe-commits]

Artem-B wrote:

> > I see one suspicious failure in tensorflow tests. I suspect I've messed something up in v4i8 comparison.
> 
> Yup, there is a problem:
> 
> ```
> Successfully custom legalized node
>  ... replacing: t10: v4i8 = BUILD_VECTOR Constant:i16<-128>, Constant:i16<-128>, Constant:i16<-128>, Constant:i16<-128>
>      with:      t17: v4i8 = bitcast Constant:i32<-128>
> ```

Resolved by https://github.com/llvm/llvm-project/pull/67866/commits/9821e908e676d8eedbee7c07c90fb5aae4454f82

https://github.com/llvm/llvm-project/pull/67866