Review and simplify `unrollType` uses #15736

Job	Run time
regression (ubuntu-latest, ocaml-variants.4.14.2+options,ocaml-option-flambda)	7m 45s
regression (macos-latest, ocaml-variants.4.14.2+options,ocaml-option-flambda)	19m 31s
extraction	0s
	27m 16s

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