Factor out common loop for copying MutableHeapNumbers in newly copied
objects.

Bug: v8:10763
Change-Id: I6cf2fc52c9ac72253dceaba518deacfde9905a9c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2402035
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Victor Gomes <victorgomes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69825}