Recent changes to the wasm format prevent you from knowing
the function index of a direct call until you know how many
imports you have (the index spaces have been merged).

Asm.js validation can't know this until all callsites have been
visited if you want to avoid materializing the entire AST / parsing twice.
Instead, keep a list of fixups to perform for all direct callsites,
then update these with the imports count added when emitting the module.

BUG=v8:4203
R=aseemgarg@chromium.org,titzer@chromium.org

Review-Url: https://codereview.chromium.org/2384623003
Cr-Commit-Position: refs/heads/master@{#39937}