This CL introduces the negb and negw instructions (8-bit and 16-bit
versions of neg) in the x64 assembler. These instructions are needed to
implement I32AtomicSub8U and similar WebAssembly instructions
efficiently.

The existing implementation was embedded in a generic macro, and it was
difficult to change it without introducing also the 8-bit and 16-bit
versions of many other instructions. This would have introduced a lot
of dead code. Instead this CL extracted the neg instructions from the
macro and implements them directly. This should be fine because the
assembler does not change much, and approachability of the code is
improved.

R=clemensb@chromium.org

Bug: v8:10108
Change-Id: I46099bbebd47f864311a67da3ba8ddc4fe4cd35d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2019165
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65989}