This CL fixes the pathological O(n^2) patching behavior that
was introduced when simplifying the wasm instance/context data
structures. It introduces a per-instance reverse mapping of
function indexes to where they appear in import and indirect
function tables. The mapping is created lazily and rebuild in
response to too many failed lookups, which makes it robust
to table mutations in the future.

This CL also fixes a bug where the anonymous lazy compile stub
was not being used for direct calls, confusing the indirect
call patching mechanism.

R=clemensh@chromium.org,mstarzinger@chromium.org

Bug: v8:7424, chromium:830558
Change-Id: Ice0212593b31eb64687a3d52bd238020682a857f
Reviewed-on: https://chromium-review.googlesource.com/1004294
Commit-Queue: Ben Titzer <titzer@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52526}