Normalize the target object if we're guaranteed to end up in
dicionary mode after copying over all enumerable properties from the
source object.

This partially addresses performance issues when using the spread
operator on large dict-mode objects.

Change-Id: I5be7cbc6e6a7e4b1b59412779db9b651f5f68b45
Bug: v8:10763
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2362961Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69713}