This adds test coverage for the source position tracking of function
table calls in asm.js and fixes the discovered issues. It also fixes
function start positions (used by errors thrown at stack checks).

R=clemensh@chromium.org
TEST=mjsunit/wasm/asm-wasm-stack
BUG=v8:6127,v8:6166

Change-Id: Id6ab6dc72bcedb0d838eed315e2a05fbc59039f4
Reviewed-on: https://chromium-review.googlesource.com/465949
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44348}