Loop headers contain a stack check in wasm, hence an exception can be
thrown at the position of the loop instruction. This means that for
asm.js, we need to store a source position for each loop instruction.

R=mstarzinger@chromium.org

Bug: chromium:799690
Change-Id: I129abef11461992e2f10af8e6afc28ce1cf83341
Reviewed-on: https://chromium-review.googlesource.com/856338Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50443}