The code generated for ChangeUint64ToBigInt in the
EffectControlLinearizer did not initialize the optional padding
field of newly allocated BigInts. This padding field is present
on 64 bit builds without pointer compression enabled. This CL
fixes this by 0-filling the padding field if present.

Bug: v8:9407
Change-Id: I511e163e676dc966a3eb6dfb92b5065e36329225
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1695464Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Nico Hartmann <nicohartmann@google.com>
Cr-Commit-Position: refs/heads/master@{#62683}