On x64 the {kScratchRegister} cannot be held in a {LiftoffRegister},
since it is not a valid cache register. Also, the code unnecessarily
checked whether there is an unused cache register, but then didn't use
it. Simplify the logic to always use the scratch register, just
distinguish between 4-byte and 8-byte moves.
On ia32 we did not move 64-bit values correctly if we didn't have
unused registers and needed to move via the stack.

R=titzer@chromium.org

Bug: v8:6600, chromium:917588, chromium:917450
Change-Id: I0bbe946c6ac8fca62f85711ae47afdac9c02ae6b
Reviewed-on: https://chromium-review.googlesource.com/c/1391755Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58521}