Simplify witness invariants for bitfield domain #14871

Job	Run time
regression (ubuntu-latest, ocaml-variants.4.14.2+options,ocaml-option-flambda)	7m 37s
regression (macos-latest, ocaml-variants.4.14.2+options,ocaml-option-flambda)	14m 49s
extraction	0s
	22m 26s

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