This reverts commit 01db326c.

Reason for revert:
https://build.chromium.org/p/client.v8/builders/V8%20Win64%20-%20msvc/builds/1607

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}

TBR=mstarzinger@chromium.org,clemensh@chromium.org

Change-Id: Id3870e671c106644b62353c2b6c0ec2607596166
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:7310
Reviewed-on: https://chromium-review.googlesource.com/939901Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51614}