The existing Load/StoreContextElement operations take the index as an int.  This
CL adds versions that take the index as a Node.  These already existed in the
interpreter-assembler, from which they are now removed.

R=mstarzinger@chromium.org, rmcilroy@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2473003004
Cr-Commit-Position: refs/heads/master@{#40810}