Swapping goto_programt::instructiont must preserve labels

The swap method should maintain all information that cannot be computed.
(As those are computed, instruction numbers, incoming edges, target
numbers, loop numbers need not be preserved.)

Author: tautschnig

regression/goto-instrument/labels1/main.c

@@ -0,0 +1,32 @@
+#include <assert.h>
+#include <stdlib.h>
+
+int foo()
+{
+  return 1;
+}
+
+typedef int (*fptr_t)(void);
+
+int main()
+{
+label_zero:
+  assert(1);
+  int x;
+  int *p;
+  goto label_two;
+label_one:
+  assert(x < 0 || x >= 0);
+label_two:
+  p = malloc(sizeof(int));
+label_three:
+  if(foo())
+    x = 42;
+label_four:
+  assert(foo() == 1);
+label_five:
+  fptr_t fp = foo;
+  assert(fp() == 1);
+label_six:
+  return *p;
+}

regression/goto-instrument/labels1/test.desc

@@ -0,0 +1,16 @@
+CORE
+main.c
+
+Labels: label_zero$
+Labels: label_one$
+Labels: label_two$
+Labels: label_three$
+Labels: label_four$
+Labels: label_five$
+Labels: label_six$
+^EXIT=0$
+^SIGNAL=0$
+--
+^warning: ignoring
+--
+This test ensures that label names specified in the source code are preserved.

src/goto-programs/goto_program.h

@@ -527,6 +527,7 @@ class goto_programt
       swap(instruction._type, _type);
       swap(instruction.guard, guard);
       swap(instruction.targets, targets);
+      swap(instruction.labels, labels);
     }
 
     /// Uniquely identify an invalid target or location