This changes DoubleToIStub to return its result on the stack instead
of a specific return register.

In a follow-up, the DoubleToIStub could be converted into a builtin.

Bug: v8:6666
Change-Id: I7852e1586c8f7b56bc5d2545a7bf6238dd2ad650
Reviewed-on: https://chromium-review.googlesource.com/1009702
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52587}