fix insertBefore / implement insertAfter

[wangzun66]

• insertBefore

• insert a stmt before a block head stmt with same trap-map
• insert a stmt before a block head stmt with different trap-map
• insert a stmt before a block middle/tail stmt with same trap-map
• insert a stmt before a block middle/tail stmt with different trap-map

• insertAfter

• insert a stmt after a block middle/head stmt with same trap-map
• insert a stmt after a block middle/head stmt with different trap-map
• insert a stmt after a block tail stmt with same trap-map
• insert a stmt after a block tail stmt with different trap-map

• other problems

• P1: After insertion, Blocks are not merged.
• P2: After insertion, Information of Traps may be false.
• P3: Sometimes(maybe with traps), a block could have two same predecessors.