Cross-instance calls call through a wasm-to-wasm stub, which
tail-calls and hence does not show up on the stack. It was not being
patched so far, leading to repeatedly calling through the
WasmCompileLazy stub. Even though this did not crash, it resulted in
significant overhead.
This CL fixes this and also adds checks to ensure that we patch at
least one call site whenever we execute the WasmCompileLazy stub.

R=titzer@chromium.org

Bug: chromium:788441, v8:5991
Change-Id: I1c2cd52497c577252a64dbf1cfa92d2f2e60b06c
Reviewed-on: https://chromium-review.googlesource.com/794132Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49709}