Fix HGraphBuilder::BuildAddStringLengths

length == String::kMaxLength is fine and should not bail out.

BUG=chromium:357052
LOG=n
R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/222113002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20433 ce2b1a6d-e550-0410-aec6-3dcde31c8c00