Layout the code for trapping arithmetic so that the overflow case comes after

the normal case.

Before, for this:

$ cat t.c
int test(int x) { return x * 2; }

We would get this:

   addl  %edi, %edi
   jno   LBB0_2
## BB#1:                                ## %overflow
   ud2
LBB0_2:                                 ## %nooverflow
   movl    %edi, %eax
   popq    %rbp
   ret

Now we get this:

   addl   %edi, %edi
   jo     LBB0_2
## BB#1:                                ## %nooverflow
   movl                                 %edi, %eax
   popq                                 %rbp
   ret
LBB0_2:                                 ## %overflow
   ud2

<rdar://problem/8283919>

llvm-svn: 134642