This is a preparation for a larger CL that needs VectorSlotPair
throughtout the compilation chain (including deoptimizer.cc).

Bug: v8:7127
Change-Id: Ia746805ca3fa294eedba19d23656f858840cd501
Reviewed-on: https://chromium-review.googlesource.com/813934Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49928}