Reader: add basic aliasing annotations

[AndyAyersMS]

In some cases we know loads can't be aliased. We should annotate the IR to reflect this.

See for instance array length load, string length load, indirections accessing runtime data, handle embedding, more handle embedding, and module domain ids.

There may be other cases of this too, I'll update as I find them.

[JosephTremoulet]

I'm looking into using !invariant.load for #257; I think that will map well to what these cases are looking for.

[AndyAyersMS]

Agreed.