Fix c2po witness invariant generation #14414

Job	Run time
regression (ubuntu-22.04, ocaml-variants.4.14.2+options,ocaml-option-flambda)	7m 28s
regression (macos-13, ocaml-variants.4.14.2+options,ocaml-option-flambda)	26m 22s
extraction	0s
	33m 50s

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