[BOLT] Do not process empty functions.

Summary:
While running on a recent test binary BOLT failed with an error. We were
trying to process '__hot_end' (which is not really a function), and asserted
that it had no basic blocks.

This diff marks functions with empty basic blocks list as non-simple since
there's no need to process them.

(cherry picked from FBD4696517)

bolt/BinaryFunction.cpp

@@ -1612,6 +1612,11 @@ bool BinaryFunction::buildCFG() {
     }
   }
 
+  if (BasicBlocks.empty()) {
+    setSimple(false);
+    return false;
+  }
+
   // Intermediate dump.
   DEBUG(print(dbgs(), "after creating basic blocks"));