[TVM] Fix negating undefined in DetectLinearEquation (#1816)

src/arithmetic/detect_linear_equation.cc

@@ -111,8 +111,9 @@ class LinearEqDetector
     return ComputeExpr<Add>(a, b);
   }
   Expr SubCombine(Expr a, Expr b) {
-    if (!a.defined()) return -b;
+    // Check b first in case they are both undefined
     if (!b.defined()) return a;
+    if (!a.defined()) return -b;
     return ComputeExpr<Sub>(a, b);
   }
   Expr MulCombine(Expr a, Expr b) {

tests/python/unittest/test_arith_detect_linear_equation.py

@@ -38,6 +38,10 @@ def test_multivariate():
     assert(m[2].value == 2)
     assert(m[len(m)-1].value == 2)
 
+    m = tvm.arith.DetectLinearEquation((v[0] - v[1]), [v[2]])
+    assert(m[0].value == 0)
+    assert(tvm.ir_pass.Simplify(m[1] - (v[0] - v[1])).value == 0)
+
 if __name__ == "__main__":
     test_basic()
     test_multivariate()