[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

compiler/testData/codegen/box/arrays/hashMap.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND: JS_IR
 operator fun HashMap<String, Int?>.set(index: String, elem: Int?) {
     this.put(index, elem)
 }

compiler/testData/codegen/box/arrays/kt950.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND: JS_IR
 operator fun <K, V> MutableMap<K, V>.set(k : K, v : V) = put(k, v)
 
 fun box() : String {