ARM64: Avoid iterating through all unresolved branch infos when many are pending.

Instead, inspect the label chain and delete pending information for every branch
in the chain.

R=ulan@chromium.org

Review URL: https://codereview.chromium.org/227043010

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20715 ce2b1a6d-e550-0410-aec6-3dcde31c8c00