[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/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/lower/IntrinsicifyCallsLowering.kt

@@ -260,7 +260,7 @@ class IntrinsicifyCallsLowering(private val context: JsIrBackendContext) : FileL
             put(Name.identifier("equals"), ::transformEqualsMethodCall)
         }
 
-        dynamicCallOriginToIrFunction.run {
+        dynamicCallOriginToIrFunction.apply {
             put(IrStatementOrigin.EXCL, context.intrinsics.jsNot)
 
             put(IrStatementOrigin.LT, context.intrinsics.jsLt)
@@ -296,6 +296,8 @@ class IntrinsicifyCallsLowering(private val context: JsIrBackendContext) : FileL
             put(IrStatementOrigin.POSTFIX_INCR, context.intrinsics.jsPostfixInc)
             put(IrStatementOrigin.POSTFIX_DECR, context.intrinsics.jsPostfixDec)
             put(IrStatementOrigin.GET_ARRAY_ELEMENT, context.intrinsics.jsArrayGet)
+            // TODO add a special statement origin, e.g. SET_ARRAY_ELEMENT
+            put(IrStatementOrigin.EQ, context.intrinsics.jsArraySet)
         }
     }