Invoking Goto in graph-builder-interface from inside a 'let' can cause
the number of locals between source and target ssa environment to be
different. This CL addresses this bug and adds a few unit tests.
Unfortunately, after this change we have to resort to always using
copy-constructors for SsaEnv, which might cause slowdown in decoding.

Bug: v8:9495
Change-Id: Idf5ace6c7563eff9d774d402f3a81e77959556ef
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2614062Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71993}