Move left-hand side type casts to right-hand side

Unlike `goto_convertt::remove_assignment`, the assignment handling code
in `goto_convertt::clean_expr` did not move type casts to the right-hand
side. This resulted in dump-c producing syntactically invalid code. The
modified code now uses the same approach as the `ID_assign` case of
`goto_convertt::remove_assignment` to avoid this problem.

This should address all failures of the CSmith CI job seen since March
2021.

Author: tautschnig

regression/goto-instrument/dump-side-effect/main.c

@@ -0,0 +1,17 @@
+struct S
+{
+  int bf : 2;
+};
+
+int foo()
+{
+  return 0;
+}
+
+int main()
+{
+  struct S s;
+  struct S *sp = &s;
+  if(sp[0].bf = foo())
+    return 0;
+}

regression/goto-instrument/dump-side-effect/test.desc

@@ -0,0 +1,12 @@
+CORE
+main.c
+--dump-c
+VERIFICATION SUCCESSFUL
+^EXIT=0$
+^SIGNAL=0$
+--
+^warning: ignoring
+--
+This test must verify successfully also for the output generated using dump-c,
+which previously would fail to compile for it included a type cast on a
+left-hand side.

src/goto-programs/goto_clean_expr.cpp

@@ -395,9 +395,10 @@ void goto_convertt::clean_expr(
         }
 
         // turn into code
-        code_assignt assignment;
-        assignment.lhs()=lhs;
-        assignment.rhs() = side_effect_assign.rhs();
+        exprt new_lhs = skip_typecast(lhs);
+        exprt new_rhs = typecast_exprt::conditional_cast(
+          side_effect_assign.rhs(), new_lhs.type());
+        code_assignt assignment(std::move(new_lhs), std::move(new_rhs));
         assignment.add_source_location()=expr.source_location();
         convert_assign(assignment, dest, mode);