As functions are relinked at instantiation anyway, we can just
completely skip the LinkFunctions pass at wasm compile time.
This way, we also don't need to create one placeholder per wasm
function, but we just use one placeholder everywhere. The function
index which was stored in the placeholder before is now retrieved from
the wire bytes anyway.

R=titzer@chromium.org

Review-Url: https://codereview.chromium.org/2624383003
Cr-Commit-Position: refs/heads/master@{#42285}