This is a reland of 01db326c.

Original change's description:
> [Assembler][x64] Make immediates immutable
> 
> 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/934341
> Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
> Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#51613}

Bug: v8:7310
Change-Id: I37f1bd1528c742e8df11b7f524316935dd289f00
Reviewed-on: https://chromium-review.googlesource.com/941621Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51655}