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

[Craig Topper via llvm-commits]

topperc wrote:

Can't we do the same thing with add+zext.h->padd.h? The way this is written seems to be overfitting one example test case and not solving the general case.

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