error when "r7" is used (starting in commit d85b3877)

Bug: chromium:1073270
Change-Id: I7ec8112f170b98d2edaf92bc9341e738f8de07a3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2163435Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67371}