[sledge] Drop calls to llvm.dbg.label

Summary:
They only attach debug info to labels, and have no execution
behavior. At some later point it would be good to scan for these and
gather the attached debug info.

Reviewed By: jvillard

Differential Revision: D27262516

fbshipit-source-id: 2eb91a475

sledge/cli/frontend.ml

@@ -1145,7 +1145,7 @@ let xlate_instr :
               emit_term ~prefix:(pop loc @ pre) (Term.throw ~exc ~loc)
           | ["__llair_unreachable"] -> emit_term Term.unreachable
           (* dropped / handled elsewhere *)
-          | ["llvm"; "dbg"; ("declare" | "value")]
+          | ["llvm"; "dbg"; ("declare" | "label" | "value")]
            |"llvm" :: ("lifetime" | "invariant") :: ("start" | "end") :: _
             ->
               nop ()