[multibyte] Add multibyte array load instructions.

[brendandahl]

Prototype implementation of the new multibyte array proposal that reuses the existing memory instructions.

https://github.com/WebAssembly/multibyte-array-access/blob/944d79230b336442b1f0b3978da1908d54b9f230/proposals/multibyte-array-access/Overview.md

[stevenfontanella]

(Re-add binaryen reviewers for a second look)

[kripken]

I believe we need to do something for Precompute here, like #8511

We could also optimize loads from immutable data, like ArrayGet, but can leave that for later.

[brendandahl]

I believe we need to do something for Precompute here, like #8511

We could also optimize loads from immutable data, like ArrayGet, but can leave that for later.

was just adding that... fdb61fa

[kripken]

Cool, that commit lgtm