title: the expression of label in llvm IR code
body: <p>Sometimes I found the label identifier in llvm IR is started with comma ';', such as 
  <code>; &lt;label&gt; 6</code>
  however as I kown, the comma is used for comments. So how can llvm retrive the label info in comments? Am I missing something?<br>
  One simple test is followed.<br>
  the C source file:</p>
  
  <pre><code>#include &lt;stdio.h&gt;
  
  int main()
  {
   int a;
   scanf("%d", &amp;a);
   if ( a &gt; 3)
    a *= 2;
   return 0;
  }
  </code></pre>
  
  <p>the llvm IR code generated by <a href="http://llvm.org/demo/index.cgi" rel="noreferrer">http://llvm.org/demo/index.cgi</a> (same as clang -c -emit-llvm main.c) is following:<br></p>
  
  <pre><code>; ModuleID = '/tmp/webcompile/_13654_0.bc'
  
  @.str = private unnamed_addr constant [3 x i8] c"%d\00", align 1
  
  define i32 @main() nounwind uwtable {
    %a = alloca i32, align 4
    %1 = call i32 (i8*, ...)* @__isoc99_scanf(i8* getelementptr inbounds ([3 x i8]* @.str, i64 0, i64 0), i32* %a) nounwind
    %2 = load i32* %a, align 4, !tbaa !0
    %3 = icmp sgt i32 %2, 3
    br i1 %3, label %4, label %6
  
  ; &lt;label&gt;:4                                       ; preds = %0
    %5 = shl nsw i32 %2, 1
    store i32 %5, i32* %a, align 4, !tbaa !0
    br label %6
  
  ; &lt;label&gt;:6                                       ; preds = %4, %0
    ret i32 0
  }
  </code></pre>