Make `RuboCop::AST::IfNode` support `then?` #1221

Job	Run time
Coding Style \| 3.0	33s
Prism	23s
Specs \| 2.7	38s
Specs \| 3.0	31s
Main Gem Specs \| RuboCop: master \| 2.7	2m 3s
Main Gem Specs \| RuboCop: master \| 3.0	1m 46s
Specs \| 3.1	28s
Main Gem Specs \| RuboCop: v1.61.0 \| 3.3	1m 53s
Specs \| 3.2	31s
Specs \| 3.3	26s
Specs \| head	27s
Coverage \| 2.7	51s
Specs "modern" \| 3.0	29s
Specs \| jruby	45s
Specs \| jruby-head	41s
Specs \| truffleruby	53s
Specs \| truffleruby-head	1m 11s
	14m 29s

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