[JS IR BE]: support dynamic_var["foo"] = "bar"

Used EQ origin to detect. Added a test to check dynamic_var = "bar" case
is not affected

js/js.translator/testData/box/dynamic/operationsWithAssignment.kt

@@ -21,6 +21,8 @@ fun box(): String {
     assertEquals(9.79999999999999, b)
     c += a * 3 + b / n
     assertEquals("text1318.233333333333334", c)
+    a = 14
+    assertEquals(14, a)
 
     return "OK"
 }