Need to use the kHashFieldSlot rather than kHashFieldOffset for
pointer-sized memory accesses.

(Fix for "[builtins] Migrate String.prototype.charCodeAt and String.prototype.charAt to TurboFan.")

R=bmeurer@chromium.org, epertoso@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1907393002

Cr-Commit-Position: refs/heads/master@{#35741}