The existing code supports overlapping LiftoffRegisters in I64Binops by
allocating a scratch register. Overlapping LiftoffRegisters means that
the low register of one LiftoffRegister is the high register of another
LiftoffRegister. When I64Binop is used in the implementation of atomic
instructions, no scratch register is available, so overlapping
LiftoffRegisters cannot be supported. As LiftoffRegisters are
not supposed to overlap anyways, this CL removes this special handling.

R=clemensb@chromium.org

Bug: v8:10108
Change-Id: I9ce2143b0930688bc2e6dd56e151f47fba00ec87
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2151351
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67307}