The lack of a vector slot for the keyed store operation in filling in
non-constant array literal properties led to undesirable contortions in
compilers downwind of full-codegen. The use of a single slot to initialize all
the array elements is sufficient.

BUG=

Review URL: https://codereview.chromium.org/1405503002

Cr-Commit-Position: refs/heads/master@{#31242}