[llvm] [docs][NVPTX] Use opaque pointers in IR examples (NFC) (PR #81941)

[Youngsuk Kim via llvm-commits]

JOE1994 wrote:

buildkite CI failed due to test failure of `"Clang :: CodeGen/fp16-ops-strictfp.c"`, which is not touched by this PR.

https://github.com/llvm/llvm-project/pull/81941