Use 64-bit addition if the pointer size is 8, use 32-bit addition
otherwise.

R=titzer@chromium.org
CC=​sreten.kovacevic@mips.com

Change-Id: I129cc29454726b7be45ecea13484212cc4f72eea
Reviewed-on: https://chromium-review.googlesource.com/998552Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52410}