[asan] make sure that the crash callbacks do not get merged (Chandler's idea:...

[asan] make sure that the crash callbacks do not get merged (Chandler's idea: insert an empty InlineAsm). Change the order in which the new BBs are inserted: the slow path BB is insert between old BBs, the crash BB is inserted at the end. Don't create an empty BB (introduced by recent commits). Update the test. The experimental code that does manual crash callback merge will most likely be deleted later. 

llvm-svn: 160544