skip set elem comparison when already matched

If both elements of the set have already been matched, there is no need
to compare them again.

internal/plans/objchange/compatible.go

@@ -366,6 +366,14 @@ func assertSetValuesCompatible(planned, actual cty.Value, path cty.Path, f func(
 	beqs := make([]bool, len(bs))
 	for ai, av := range as {
 		for bi, bv := range bs {
+			if aeqs[ai] && beqs[bi] {
+				// because the best we can do is check that a value correlates
+				// with some other value in the other set, one or both values
+				// might have already been matched. If they have both matched in
+				// some way there's no need run the check again.
+				continue
+			}
+
 			if f(av, bv) {
 				aeqs[ai] = true
 				beqs[bi] = true