Add test cases for return slot allocation.

test/libyul/evmCodeTransform/stackReuse/function_retparam_if.yul

@@ -0,0 +1,23 @@
+{
+    function f() -> x { pop(address()) if 1 { pop(callvalue()) } }
+}
+// ====
+// stackOptimization: true
+// ----
+// PUSH1 0x14
+// JUMP
+// JUMPDEST
+// PUSH1 0x0
+// ADDRESS
+// POP
+// PUSH1 0x1
+// ISZERO
+// PUSH1 0x10
+// JUMPI
+// CALLVALUE
+// POP
+// JUMPDEST
+// JUMPDEST
+// SWAP1
+// JUMP
+// JUMPDEST