[JS BE] Support val in when subject

#KT-25014 Fixed

compiler/testData/codegen/box/when/whenSubjectVariable/ieee754Equality.kt

@@ -1,5 +1,4 @@
 // !LANGUAGE: +VariableDeclarationInWhenSubject
-// IGNORE_BACKEND: JS
 
 val dz = -0.0
 val fz = -0.0f
@@ -29,5 +28,14 @@ fun box(): String {
         }
     }
 
+    testDoubleAsUpperBound(-0.0)
+
     return "OK"
-}
+}
+
+fun <T: Double> testDoubleAsUpperBound(v: T): Boolean {
+    return when (val a = v*v) {
+        0.0 -> true
+        else -> throw AssertionError()
+    }
+}