The Operand class is small enough to be efficiently passed by value.
This saves binary size and performance because the Operand does not need
to be emitted to the caller's frame and loaded in the callee.
Binary saving is 30kB in release mode on x64.

R=mstarzinger@chromium.org

Bug: v8:7310
Change-Id: Ibad9d438ace23cfe8e4641bd48063ab7035656f4
Reviewed-on: https://chromium-review.googlesource.com/934137
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51524}