This removes the need for certain context scopes to skip popping the
context register. For the {incoming_context} the flag was already
obsolete, because its destructor would only run once the basic block
ended with a return. For {local_function_context} the same holds now
by moving handling of implicit returns into the body visitor.

R=rmcilroy@chromium.org

Change-Id: Icceaab1b30d7223b2b2f87a092a6580be7d7d675
Reviewed-on: https://chromium-review.googlesource.com/513963Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45511}