JS: make string template optimization more conservative

See KT-18548

js/js.translator/testData/box/expression/stringTemplates/nonStrings.kt

@@ -0,0 +1,23 @@
+// EXPECTED_REACHABLE_NODES: 1024
+fun box(): String {
+    val x = foo()
+    val y = bar()
+
+    var r = "$x$y"
+    if (r != "2342") return "fail: $r"
+
+    r = "$y$x"
+    if (r != "4223") return "fail: $r"
+
+    r = "$x$x"
+    if (r != "2323") return "fail: $r"
+
+    r = "$y$y"
+    if (r != "4242") return "fail: $r"
+
+    return "OK"
+}
+
+fun foo(): Any = 23
+
+fun bar() = 42