Merge pull request #6589 from ethereum/addReturn

[Yul] Add code generation for "return" and for loops.

test/libsolidity/semanticTests/viaYul/loops.sol

@@ -0,0 +1,40 @@
+contract C {
+    function f() public returns (uint x) {
+        x = 1;
+        for (uint a = 0; a < 10; a = a + 1) {
+            x = x + x;
+        }
+    }
+    function g() public returns (uint x) {
+        x = 1;
+        for (uint a = 0; a < 10; a = a + 1) {
+            x = x + x;
+            break;
+        }
+    }
+    function h() public returns (uint x) {
+        x = 1;
+        uint a = 0;
+        for (; a < 10; a = a + 1) {
+            continue;
+            x = x + x;
+        }
+        x = x + a;
+    }
+    function i() public returns (uint x) {
+        x = 1;
+        uint a;
+        for (; a < 10; a = a + 1) {
+            return x;
+            x = x + x;
+        }
+        x = x + a;
+    }
+}
+// ===
+// compileViaYul: true
+// ----
+// f() -> 1024
+// g() -> 2
+// h() -> 11
+// i() -> 1

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