[llvm] clarify semantics of masked vector load/store (PR #82469)

[Ralf Jung via llvm-commits]

RalfJung wrote:

That doesn't quite say everything -- there's the question of whether this [Rust PR](https://github.com/rust-lang/rust/pull/119203) should say `offset` (aka `getelementptr inbounds`) or `offset_wrapping` (aka `getelementptr`) when describing how the pointers to the individual elements being loaded are computed.

https://github.com/llvm/llvm-project/pull/82469