[JS IR BE] Support 'is Number' and 'is Comparable<>'

libraries/stdlib/js/irRuntime/misc.kt

@@ -6,5 +6,5 @@
 package kotlin.js
 
 // TODO: Polyfill
-fun imul(a: dynamic, b: dynamic) =
-    js("((a & 0xffff0000) * (b & 0xffff) + (a & 0xffff) * (b | 0)) | 0")
+internal fun imul(a: Int, b: Int) =
+    js("((a & 0xffff0000) * (b & 0xffff) + (a & 0xffff) * (b | 0)) | 0").unsafeCast<Int>()

libraries/stdlib/js/irRuntime/typeCheckUtils.kt

@@ -150,4 +150,16 @@ public fun jsIsType(obj: dynamic, jsClass: dynamic): Boolean {
     }
 
     return false
-}
+}
+
+fun isNumber(a: dynamic) = jsTypeOf(a) == "number" || a is Long
+
+fun isComparable(value: dynamic): Boolean {
+    var type = jsTypeOf(value)
+
+    return type == "string" ||
+           type == "boolean" ||
+           isNumber(value) ||
+           isInterface(value, Comparable::class.js)
+}
+