This CL adds a speculative operator for BigInt negation that is
lowered to the respective builtin call and is optimized to native
64 bit machine operations if truncated. In particular, this change
allows negative BigInt constants (e.g. -5n) to be lowered.

Bug: v8:9407
Change-Id: Ia98fd6dee18a31ce56efbe537f4352b1582539e7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1695463
Commit-Queue: Nico Hartmann <nicohartmann@google.com>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62684}