SVA: sequence() methods for first_match and throughout

This adds convenience accessors for the sequence-typed operand of the SVA
first_match and throughout expression classes.

Author: kroening

src/verilog/sva_expr.h

@@ -1746,6 +1746,16 @@ class sva_sequence_throughout_exprt : public binary_exprt
     : binary_exprt(std::move(op0), ID_sva_sequence_throughout, std::move(op1))
   {
   }
+
+  const exprt &sequence() const
+  {
+    return op1();
+  }
+
+  exprt &sequence()
+  {
+    return op1();
+  }
 };
 
 static inline const sva_sequence_throughout_exprt &
@@ -1780,6 +1790,16 @@ class sva_sequence_first_match_exprt : public binary_exprt
         std::move(action))
   {
   }
+
+  const exprt &sequence() const
+  {
+    return op0();
+  }
+
+  exprt &sequence()
+  {
+    return op0();
+  }
 };
 
 static inline const sva_sequence_first_match_exprt &