Symex-dereference: simplify after deref

This is generally useful because it turns member-of-if into if-of-member, which avoids repeatedly quoting
large complex if expressions in a with_exprt, but it is especially good for field sensitivity as it puts
member expressions directly on top of symbol expressions, which field sensitivity knows how to deal with.

Author: smowton

src/goto-symex/symex_dereference.cpp

@@ -404,5 +404,14 @@ void goto_symext::dereference(
   // dereferencing may introduce new symbol_exprt
   // (like __CPROVER_memory)
   state.rename(expr, ns, goto_symex_statet::L1);
+
+  // dereferencing is likely to introduce new member-of-if constructs --
+  // for example, "x->field" may have become "(x == &o1 ? o1 : o2).field"
+  // Simplify (which converts that to (x == &o1 ? o1.field : o2.field)) before
+  // applying field sensitivity, which can turn such field-of-symbol expressions
+  // into atomic SSA expressions, but would have to rewrite all of 'o1'
+  // otherwise.
+  do_simplify(expr);
+
   field_sensitivityt::apply(ns, expr, write, state);
 }