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

Committed: https://crrev.com/17add22ff4b9c5ca638502e7708f0d9d99baca40
Cr-Commit-Position: refs/heads/master@{#29543}

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

Cr-Commit-Position: refs/heads/master@{#29588}