Fix infinite loop when accessing circular constants from inline assembly.

test/libsolidity/syntaxTests/inlineAssembly/circular_module_access_err.sol

@@ -0,0 +1,14 @@
+==== Source: a ====
+bytes32 constant x = x;
+==== Source: b ====
+import "a";
+contract C {
+    function f() public pure returns (uint t) {
+        assembly {
+            // Reference to a circular member
+            t := x
+        }
+    }
+}
+// ----
+// TypeError 3558: (b:155-156): Constant variable is circular.