This avoids emitting redundant {PopContext} bytecode instructions when
non-local control-flow leaves the method body. It also folds multiple
such {PopContext} instructions into one, in case several scoping levels
are crossed at one. Only the expected context of the target of a local
control-flow transfer matters.

R=rmcilroy@chromium.org
TEST=debugger/regress/regress-crbug-724858
BUG=chromium:724858

Change-Id: Id4a47ae9fea25e75ae1af13619720b16a3975edf
Reviewed-on: https://chromium-review.googlesource.com/512545Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45507}