Introduce the CheckString during native context specialization when we
have string map feedback on a LOAD_IC/STORE_IC. The CheckString
operator, just like its CheckNumber pendant, renames the input and
assigns a proper type, which we will use soon to lower access operations
on Strings, i.e. charCodeAt calls or keyed accesses.

R=jarin@chromium.org
BUG=v8:4930,v8:5141

Review-Url: https://codereview.chromium.org/2181943003
Cr-Commit-Position: refs/heads/master@{#38076}