Correct the simd_masked_{load,store} intrinsic docs

[farnoy]

Explains the uniform pointer being used for these two operations and how elements are offset from it.

[rustbot]

r? @thomcc

(rustbot has picked a reviewer for you, use r? to override)

[RalfJung]

@rustbot author

[farnoy]

@rustbot ready

I will squash the commits when it's approved

[RalfJung]

LGTM, r=me after squashing.

@bors delegate+

[bors]

:v: @farnoy, you can now approve this pull request!

If @RalfJung told you to "r=me" after making some further change, please make that change, then do @bors r=@RalfJung

[thomcc]

I'm going to be away for a few months, so I'm rerolling my PRs so that folks don't have to wait for me. Sorry/thanks.

r? libs

[RalfJung]

r? @RalfJung

@farnoy this is just waiting for you to squash the commits. :)

[rustbot]

Some changes occurred to the platform-builtins intrinsics. Make sure the LLVM backend as well as portable-simd gets adapted for the changes.

cc @antoyo, @GuillaumeGomez, @bjorn3, @calebzulawski, @programmerjake

[farnoy]

Missed the notifications.

@bors r=@RalfJung

[bors]

:pushpin: Commit 14a45516951edbbc9dd404529073dc9e9dd40351 has been approved by RalfJung

It is now in the queue for this repository.

[saethlin]

@bors rollup=always (docs-only change)