[TableGen] Change to attach the name to DAG operator in result patterns

There are two ways that we can attach a name to a DAG node:

1) (Op:$name ...)
2) (Op ...):$name

The problem with 2) is that we cannot do it on the outmost DAG node in a tree.
Switch from 2) to 1).

PiperOrigin-RevId: 237513962