Merged r17432, r17526, r17545, r17620, r17747 into 3.21 branch.

Do not remove HAdd with zero if the other operand is a double.

Age ICs on context dispose notification.

Fix y-umlaut to uppercase.

Fix duplicate check in DependentCode::Insert.

BUG=309723,v8:2984,318454
R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/80953002

git-svn-id: https://v8.googlecode.com/svn/branches/3.21@17971 ce2b1a6d-e550-0410-aec6-3dcde31c8c00