Drive-by: Fix ExternalReferenceAddressAsOperand. Now it correctly
returns an operand to within the external reference table.

Bug: v8:6666
Change-Id: I8226257f27a2747584d37fd948d66c6e9b0d7ecf
Reviewed-on: https://chromium-review.googlesource.com/c/1286671
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56739}