core: Fix terminator conditions for unregistered operations (#1201)

This PR refines terminator conditions for unregistered operations.
Namely, if we allow unregistered operations, then their terminator
conditions are implicitly fulfilled.
It also modifies `has_trait` to assume the existence of any trait for
unregistered operations.

This is modelled after the [`mayBeValidWithoutTerminator`][2] and
[`mightHaveTrait`][1] in MLIR.


Resolves #1167

[1]:
https://github.com/llvm/llvm-project/blob/268032f6f10d595bb723f6b4a21632a9f3b35be8/mlir/include/mlir/IR/OperationSupport.h#L288
[2]:
https://github.com/llvm/llvm-project/blob/268032f6f10d595bb723f6b4a21632a9f3b35be8/mlir/lib/IR/Verifier.cpp#L100

tests/filecheck/parser-printer/unregistered_dialect.mlir

@@ -6,14 +6,12 @@
     %y = "op_with_res"() {otherattr = #unknown_attr<b2 ...2 [] <<>>>} : () -> (i32)
     %z = "op_with_operands"(%y, %y) : (i32, i32) -> !unknown_type<{[<()>]}>
     "op"() {ab = !unknown_singleton_type} : () -> ()
-    "test.termop"() : () -> ()
   }) {testattr = "foo"} : () -> i32
 
   // CHECK:       %{{.*}} = "region_op"() ({
   // CHECK-NEXT:   %{{.*}} = "op_with_res"() {"otherattr" = #unknown_attr<b2 ...2 [] <<>>>} : () -> i32
   // CHECK-NEXT:   %{{.*}} = "op_with_operands"(%{{.*}}, %{{.*}}) : (i32, i32) -> !unknown_type<{[<()>]}>
   // CHECK-NEXT:   "op"() {"ab" = !unknown_singleton_type} : () -> ()
-  // CHECK-NEXT:   "test.termop"() : () -> ()
   // CHECK-NEXT:  }) {"testattr" = "foo"} : () -> i32
 
 }) : () -> ()

xdsl/ir/core.py

@@ -899,10 +899,22 @@ def clone(
         return op
 
     @classmethod
-    def has_trait(cls, trait: type[OpTrait], parameters: Any = None) -> bool:
+    def has_trait(
+        cls,
+        trait: type[OpTrait],
+        parameters: Any = None,
+        value_if_unregistered: bool = True,
+    ) -> bool:
         """
         Check if the operation implements a trait with the given parameters.
+        If the operation is not registered, return value_if_unregisteed instead.
         """
+
+        from xdsl.dialects.builtin import UnregisteredOp
+
+        if issubclass(cls, UnregisteredOp):
+            return value_if_unregistered
+
         return cls.get_trait(trait, parameters) is not None
 
     @classmethod