Behavior in Release mode was correct; Debug mode triggered a DCHECK
failure that indicated an inefficiency: when a requested truncation
would be a no-op, we should return the original BigInt, rather than
creating a copy. In the special case of -2^(n-1), i.e. the smallest
negative n-bit integer, getting truncated to n bits, with n being a
multiple of kDigitBits, this shortcut was not taken.

Bug: v8:8426
Change-Id: I8e4595d9ac0dbef81aae06688f9a9636bd2d9cd9
Reviewed-on: https://chromium-review.googlesource.com/c/1325029Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57369}