If StoreIC stores into a kConst field, only take the slow path
if the value is different from the current value.

This recovers the ObjectAssign regression in crbug.com/930680.

Bug: v8:8361, chromium:930680
Change-Id: Ie27d15d624b07ab1dcb58b244a46b87eec34bd0f
Reviewed-on: https://chromium-review.googlesource.com/c/1470134
Commit-Queue: Jaroslav Sevcik <jarin@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59618}