Implement return.

test/libsolidity/semanticTests/viaYul/return.sol

@@ -0,0 +1,10 @@
+contract C {
+    function f() public pure returns (uint x) {
+        return 7;
+        x = 3;
+    }
+}
+// ====
+// compileViaYul: true
+// ----
+// f() -> 7

test/libsolidity/semanticTests/viaYul/return_and_convert.sol

@@ -0,0 +1,11 @@
+contract C {
+    function f() public pure returns (uint) {
+        uint8 b;
+        assembly { b := 0xffff }
+        return b;
+    }
+}
+// ====
+// compileViaYul: true
+// ----
+// f() -> 255