Instead of storing call_pc and ret_pc, store just one pc. This
simplifies frame inspection (no distinction between top frame and other
frames). The previous ret_pc can easily be computed from the stored pc,
since we know that we must be at a indirect or direct call site when
returning to a previous frame.
It also slightly simplifies the upcoming CL to call imported functions,
which would also have to set the call_pc.

R=titzer@chromium.org, ahaas@chromium.org
BUG=v8:5822

Change-Id: I5876c09ec36450dc1474a760282fd5e41eab38be
Reviewed-on: https://chromium-review.googlesource.com/453159
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43777}