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 37kB in release mode on ia32.

R=mstarzinger@chromium.org

Bug: v8:7310
Change-Id: Ibc103622ec216725c762c2ba4bb96451c99db556
Reviewed-on: https://chromium-review.googlesource.com/934264
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51555}