Intrinsify String?.plus in JS BE

See KT-8020

js/js.translator/testData/box/standardClasses/stringPlus.kt

@@ -0,0 +1,51 @@
+// EXPECTED_REACHABLE_NODES: 489
+
+fun box(): String {
+    var x: String? = foo()
+    var r = x + bar()
+    if (r != "foobar") return "fail1: $r"
+
+    x = null
+    r = x + bar()
+    if (r != "nullbar") return "fail2: $r"
+
+    x = foo()
+    r = x + null
+    if (r != "foonull") return "fail3: $r"
+
+    x = foo()
+    r = x + nullString()
+    if (r != "foonull") return "fail4: $r"
+
+    r = foo()
+    r += bar()
+    if (r != "foobar") return "fail5: $r"
+
+    x = null
+    r = x + null
+    if (r != "nullnull") return "fail6: $r"
+
+    x = foo()
+    x += nullString()
+    if (x != "foonull") return "fail7: $r"
+
+    x = nullString()
+    x += bar()
+    if (x != "nullbar") return "fail8: $r"
+
+    x = nullString()
+    r = x + nullString()
+    if (r != "nullnull") return "fail9: $r"
+
+    x = nullString()
+    x += nullString()
+    if (x != "nullnull") return "fail10: $x"
+
+    return "OK"
+}
+
+fun foo() = "foo"
+
+fun bar() = "bar"
+
+fun nullString(): String? = null