Add test cases for return slot allocation.

test/libyul/evmCodeTransform/stackReuse/function_retparam_leave.yul

@@ -0,0 +1,20 @@
+{
+    function f() -> x { pop(address()) leave pop(callvalue()) }
+}
+// ====
+// stackOptimization: true
+// ----
+// PUSH1 0x10
+// JUMP
+// JUMPDEST
+// PUSH1 0x0
+// ADDRESS
+// POP
+// PUSH1 0xD
+// JUMP
+// CALLVALUE
+// POP
+// JUMPDEST
+// SWAP1
+// JUMP
+// JUMPDEST