[turbofan] Optimize string "length" property access based on types.

[turbofan] Optimize string "length" property access based on types.

Optimize string "length" property access based on static type
information if possible, but also optimistically optimize the access
based on type feedback from the LoadIC.

R=jarin@chromium.org

Review URL: https://codereview.chromium.org/1216593003

Cr-Commit-Position: refs/heads/master@{#29543}