[llvm] [NVPTX] Improve lowering of v4i8 (PR #67866)

[Artem Belevich via llvm-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>
```


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