Be careful with predecessor/successor index.

When we simplify the CFG, we must preserve things that were already
simplified. For example, the index in the predecessor list or
successor list of a block must be preserved for ensuring the
first block is a loop pre header.

bug:21867463

Change-Id: Ic3fcb3eb2c3fb109d8a57ee2a6b6d4d65fdb9410