Updates to control structures (2).

Co-Authored-By: Bhargava Shastry <bhargava.shastry@ethereum.org>

test/libsolidity/semanticTests/tryCatch/assert.sol

@@ -0,0 +1,16 @@
+contract C {
+    function g(bool x) public pure {
+        assert(x);
+    }
+    function f(bool x) public returns (uint) {
+        // Set the gas to make this work on pre-byzantium VMs
+        try this.g.gas(8000)(x) {
+            return 1;
+        } catch {
+            return 2;
+        }
+    }
+}
+// ----
+// f(bool): true -> 1
+// f(bool): false -> 2