[TableGen] Improve handling for dag op names (#149248)
There are currently no ways to add names to dag
operators other than when defining them. Furthermore a !con operation as
well as some others, drops the operator names.
This patch propagates the name from the LHS dag
for !con and adds a way to get and set the operator name for a dag
(!getdagopname, !setdagopname).
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Co-authored-by: Nemanja Ivanovic <nemanja@synopsys.com>
