Do not generate redundant if statements in assert expansions

Fixes: diffblue#755

Author: tautschnig

regression/goto-instrument/assert1/main.c

@@ -0,0 +1,7 @@
+#include <assert.h>
+
+int main(int argc, char *argv[])
+{
+  assert(0);
+  return 0;
+}

regression/goto-instrument/assert1/test.desc

@@ -0,0 +1,8 @@
+CORE
+main.c
+--dump-c
+^EXIT=10$
+^SIGNAL=0$
+--
+^warning: ignoring
+^[[:space:]]*IF

src/goto-programs/goto_convert.cpp

@@ -26,6 +26,7 @@ Author: Daniel Kroening, [email protected]
 
 #include "goto_convert_class.h"
 #include "destructor.h"
+#include "remove_skip.h"
 
 static bool is_empty(const goto_programt &goto_program)
 {
@@ -1825,6 +1826,11 @@ void goto_convertt::generate_ifthenelse(
     true_case.instructions.back().guard=boolean_negate(guard);
     dest.destructive_append(true_case);
     true_case.instructions.clear();
+    if(
+      is_empty(false_case) ||
+      (is_size_one(false_case) &&
+       is_skip(false_case, false_case.instructions.begin())))
+      return;
   }
 
   // similarly, do guarded assertions directly
@@ -1838,6 +1844,11 @@ void goto_convertt::generate_ifthenelse(
     false_case.instructions.back().guard=guard;
     dest.destructive_append(false_case);
     false_case.instructions.clear();
+    if(
+      is_empty(true_case) ||
+      (is_size_one(true_case) &&
+       is_skip(true_case, true_case.instructions.begin())))
+      return;
   }
 
   // Flip around if no 'true' case code.