Fix a potential bug in r183584.

  r183584 tries to derive some info from the code *AFTER* a call and apply
these derived info to the code *BEFORE* the call, which is not always safe
as the call in question may never return, and in this case, the derived
info is invalid.
  
  Thank Duncan for pointing out this potential bug.

rdar://14073661 

llvm-svn: 183606