JS: -fcheck-prim-bounds is completely broken for ByteArray# ops

Summary

Although there are several problems, the biggest is that the code emitted for the checks looks at the length field when the ByteArray# objects store their sizes in the len field. JS being JS, this causes most bounds-checks to silently result in "out-of-bounds", with no indication that something else is wrong.

(forked from !9629 (comment 484210))