Collect string feedback for compare operations. Without this,
functions which have a lot of string compare operations end up with
a high generic type percentage, and don't get optimized until very
late.

Currently TurboFan doesn't use this String feedback for compare
operations, but this could be done in future work if it is useful.

BUG=chromium:660947

Review-Url: https://codereview.chromium.org/2506013005
Cr-Commit-Position: refs/heads/master@{#41078}