llgo/ssa: phi refactor

ssa/ssa_test.go

@@ -109,9 +109,7 @@ func TestCvtType(t *testing.T) {
 }
 
 func TestUserdefExpr(t *testing.T) {
-	b := &phisExprTy{}
 	c := &pyVarTy{}
-	_ = b.String()
 	_ = c.String()
 	test := func(a types.Type) {
 		defer func() {
@@ -121,7 +119,6 @@ func TestUserdefExpr(t *testing.T) {
 		}()
 		a.Underlying()
 	}
-	test(b)
 	test(c)
 }