[sledge] Model llvm.trap as nop

Summary:
llvm.trap is noreturn nounwind so calls to it are always succeeded by
Unreachable, therefore unless an alarm is desired for reaching it,
translating it as nop suffices.

Reviewed By: ngorogiannis

Differential Revision: D15328302

fbshipit-source-id: 54efe6c21

sledge/src/llair/frontend.ml

@@ -952,7 +952,8 @@ let xlate_instr :
           emit_inst (Llair.Inst.memmov ~dst ~src ~len ~loc)
       (* dropped / handled elsewhere *)
       | ["llvm"; "dbg"; ("declare" | "value")]
-       |"llvm" :: ("lifetime" | "invariant") :: ("start" | "end") :: _ ->
+       |"llvm" :: ("lifetime" | "invariant") :: ("start" | "end") :: _
+       |["llvm"; "trap"] ->
           nop ()
       (* unimplemented *)
       | ["llvm"; ("stacksave" | "stackrestore")] ->