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

(cherry picked from commit 8b20f88b)

Change-Id: I2581b5a50942290da96cd9ec876f6f2573e0a6c4