[llvm] [LangRef] Rename 'operand' to 'argument' descriptions of VP intrinsics. NFC (PR #97437)

[Fraser Cormack via llvm-commits]

https://github.com/frasercrmck approved this pull request.

Generally seems like a good idea, thanks. Only the `llvm.experimental.gc.*` intrinsics seem to explicitly label their sub-section "Operands". That can maybe be cleaned up too, in a separate PR, for consistency.

I'm still a bit surprised that instructions have "arguments" but that's personal preference.

https://github.com/llvm/llvm-project/pull/97437