[sledge] fix LLVM assertion failure in xlate_global

Summary:
LLVM.global_initializer casues a cast assertion failure if the value
passed to it is not a GlobalVariable. So we first check if it is a
GlobalVariable and only then ask for an initiliazer.

This is hidden if LLVM is built without assertions.

Reviewed By: jberdine

Differential Revision: D15601632

fbshipit-source-id: e9db23a12

sledge/src/llair/frontend.ml

@@ -588,11 +588,14 @@ and xlate_global : x -> Llvm.llvalue -> Global.t =
          its own initializer *)
       Hashtbl.set memo_global ~key:llg ~data:(Global.mk g typ loc) ;
       let init =
-        match (Llvm.classify_value llg, Llvm.global_initializer llg) with
-        | GlobalVariable, Some llinit ->
+        match Llvm.classify_value llg with
+        | GlobalVariable -> (
+          match Llvm.global_initializer llg with
+          | Some llinit ->
               let siz = size_of x (Llvm.element_type llt) in
               let init = xlate_value x llinit in
               Some (init, siz)
+          | _ -> None )
         | _ -> None
       in
       Global.mk ?init g typ loc