Fix RubyVM::Shape.transition_tree #21

Job	Run time
omnibus compilations, trigger	2s
omnibus compilations, #2	32m 46s
omnibus compilations, #1	27m 49s
omnibus compilations, #4	26m 11s
omnibus compilations, #3	30m 20s
omnibus compilations, #8	5m 16s
omnibus compilations, #7	32m 40s
omnibus compilations, #6	27m 11s
omnibus compilations, #12	15m 16s
omnibus compilations, #9	17m 41s
omnibus compilations, #5	26m 5s
omnibus compilations, #11	26m 54s
omnibus compilations, #10	26m 15s
omnibus compilations, result	10s
	4h 54m 36s

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