Cannot break a chain to preserve WAR dependencies in pipelined block.

[etherzhhb]

After chaining is enabled, the result of an operation in a multi-cycles chain may be read at the middle of a cycles. This means we cannot break the chains at a arbitrary position, hence sometime we cannot break a chain to preserve WAR dependencies in pipelined block. A fail case is shown in [1].

[1]http://192.168.1.253:8010/builders/CustomFast/builds/24