[llvm] [RISCV] Use `experimental-p` extension for zero-extended narrow unsigned types (PR #213260)

[Craig Topper via llvm-commits]

topperc wrote:

What about a test like

```
define zeroext i8 @zexti8_paddb(i8 zeroext %x, i8 zeroext %y, i8 zeroext %z) {
entry:
  %add = add i8 %y, %x
  %add2 = add i8 %add, %z
  ret i8 %add2
}
```

We can use 2 padd.b to handle that, but this patch won't do it. That doesn't seem like something that can be handled from isel. Probably would need a DAGCombine.

https://github.com/llvm/llvm-project/pull/213260