Improve options printed on vectorization analysis diagnostics.

The LLVM patch changes the analysis diagnostics produced when loops with            
floating-point recurrences or memory operations are identified. The new messages
say "cannot prove it is safe to reorder * operations; allow reordering by       
specifying #pragma clang loop vectorize(enable)". Depending on the type of      
diagnostic the message will include additional options such as ffast-math or    
__restrict__.                                                                   

llvm-svn: 246189