fix(library/elaborator): missing case
fix(library/elaborator): missing case

The elaborator was failing in the following scenario:
- Failing constraint of the form
     ctx |- ?m1 =:= ?m2
where
     ?m2 is assigned to ?m1,
     and ?m1 is unassigned.

has_metavar(?m2, ?m1) returns true, and a cycle is incorrectly reported.

Signed-off-by: Leonardo de Moura <7610bae85f2b530654cc716772f1fe653373e892@microsoft.com>

sp3ctum/lean,leanprover/lean,leanprover/lean,leodemoura/lean,dselsam/lean,soonhokong/lean,avigad/lean,c-cube/lean,UlrikBuchholtz/lean,sp3ctum/lean,htzh/lean,htzh/lean,codyroux/lean0.1,leodemoura/lean,avigad/lean,c-cube/lean,soonhokong/travis_test,johoelzl/lean,fgdorais/lean,avigad/lean,leanprover/lean,Kha/lean,levnach/lean,digama0/lean,leanprover/lean,javra/lean,javra/lean,soonhokong/lean-windows,soonhokong/lean,avigad/lean,fpvandoorn/lean2,levnach/lean,UlrikBuchholtz/lean,htzh/lean,Kha/lean,digama0/lean,dselsam/lean,leodemoura/lean,UlrikBuchholtz/lean,rlewis1988/lean,soonhokong/lean-windows,fgdorais/lean,leodemoura/lean,leanprover-community/lean,javra/lean,levnach/lean,levnach/lean,soonhokong/lean,fgdorais/lean,rlewis1988/lean,eigengrau/lean,leanprover/lean,c-cube/lean,fpvandoorn/lean2,codyroux/lean0.1,eigengrau/lean,rlewis1988/lean,leanprover-community/lean,fpvandoorn/lean,dselsam/lean,soonhokong/travis_test,soonhokong/lean-osx,fpvandoorn/lean2,fpvandoorn/lean,digama0/lean,eigengrau/lean,johoelzl/lean,leanprover-community/lean,Kha/lean,fgdorais/lean,leanprover/lean,avigad/lean,soonhokong/lean-osx,sp3ctum/lean,levnach/lean,fgdorais/lean,soonhokong/lean-osx,digama0/lean,javra/lean,dselsam/lean,htzh/lean,fpvandoorn/lean,digama0/lean,UlrikBuchholtz/lean,soonhokong/lean-windows,rlewis1988/lean,soonhokong/lean-windows,codyroux/lean0.1,digama0/lean,Kha/lean,leanprover-community/lean,avigad/lean,htzh/lean,soonhokong/lean-osx,Kha/lean,johoelzl/lean,johoelzl/lean,fpvandoorn/lean,soonhokong/lean-osx,leanprover-community/lean,dselsam/lean,leanprover-community/lean,dselsam/lean,johoelzl/lean,rlewis1988/lean,fpvandoorn/lean,johoelzl/lean,sp3ctum/lean,Kha/lean,fpvandoorn/lean2,soonhokong/travis_test,codyroux/lean0.1,soonhokong/travis_test,soonhokong/lean,sp3ctum/lean,eigengrau/lean,soonhokong/lean-windows,leodemoura/lean,c-cube/lean,fpvandoorn/lean,soonhokong/lean,c-cube/lean,fgdorais/lean,fpvandoorn/lean2,javra/lean,rlewis1988/lean,leodemoura/lean,eigengrau/lean,UlrikBuchholtz/lean