This makes sure that function imports without a single call site within
the asm.js module are still preserved in the WebAssembly module, hence
preserving intended JavaScript semantics during module instantiation.

R=clemensh@chromium.org
TEST=mjsunit/regress/regress-crbug-722348
BUG=chromium:722348

Change-Id: I624d0e52b32b864c1e3002187a99a0a63834a4b0
Reviewed-on: https://chromium-review.googlesource.com/509450Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45452}