This helps to debug issues with maintaining the cache state, and also
understanding errors in the generated code.

Unfortunately, it requires buffering the trace output in the decoder,
since the interface is called in between, and the output would be
messed up otherwise.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: Ie8af8f7f619f3909ea52268241b883a4d4de79fa
Reviewed-on: https://chromium-review.googlesource.com/813972
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49989}