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

[Ralf Jung via llvm-commits]

RalfJung wrote:

@nikic I have updated the wording to

> The result of this operation is equivalent to a regular vector load instruction followed by a 'select' between the loaded and the passthru values, predicated on the same mask, except that the masked-off lanes are not accessed.

Followed by clarification regarding exceptions, noalias, and data races. Does that work for you?

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