On x64, we already pass immediates by value. This CL ensures that this
is indeed cheap, and it makes immediates immutable.

R=mstarzinger@chromium.org

Bug: v8:7310
Change-Id: I53a0666d53b9de69d390621298798c03b5190497
Reviewed-on: https://chromium-review.googlesource.com/934341Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51613}