Don't double smi-tag the mask used to check read-only / field mode in the dictionary-store-stub on X64

BUG=

Review URL: https://codereview.chromium.org/757673002

Cr-Commit-Position: refs/heads/master@{#25493}
Reduce context disposal gc overhead.

BUG=

Review URL: https://codereview.chromium.org/750963002

Cr-Commit-Position: refs/heads/master@{#25494}