[runtime/heap] Introduce CopyFixedArrayUpTo to match CopyFixedArrayAndGrow, copying to a smaller array.

This allows the helper to avoid write barriers while copying, speeding up Object.keys by 5-10%.

BUG=

Review URL: https://codereview.chromium.org/1690953002

Cr-Commit-Position: refs/heads/master@{#33916}