Replacing a constant BigInt with a constant int64 is only valid
when the use site has truncating semantics. (For non-constant
values, the representation changer did correctly check for this.)

Bug: chromium:1028593
Change-Id: Ib58b16ece6f21ba30153fd6cfa0560cc2d78d6a2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1940263Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65262}