The Cvtui2ss method did overwrite the {src} register, and the given
{tmp} register. Because of this, the Turbofan code generator passed two
temporary registers.
This CL fixes this to avoid the overwrite of the {src} register (which
is now an Operand).

R=neis@chromium.org

Change-Id: I33e523ac3d7bb377899739e95058b87adefa6b65
Reviewed-on: https://chromium-review.googlesource.com/1014082
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52625}