[a64] Omit redundant smi checks in write barriers.

R=rmcilroy@chromium.org

Review URL: https://codereview.chromium.org/156603007

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19324 ce2b1a6d-e550-0410-aec6-3dcde31c8c00