Generate type check predicates for secondary type variables.

The encoding tables are keyed by the controlling type variable only. We
need to distinguish different encodings for instructions with multiple
type variables.

Add a TypePredicate instruction predicate which can check the type of an
instruction value operand. Combine type checks into the instruction
predicate for instructions with more than one type variable.

Add Intel encodings for fcvt_from_sint.f32.i64 which can now be
distinguished from fcvt_from_sint.f32.i32.

lib/cretonne/meta/cdsl/formats.py

@@ -11,6 +11,29 @@ except ImportError:
     pass
 
 
+class InstructionContext(object):
+    """
+    Most instruction predicates refer to immediate fields of a specific
+    instruction format, so their `predicate_context()` method returns the
+    specific instruction format.
+
+    Predicates that only care about the types of SSA values are independent of
+    the instruction format. They can be evaluated in the context of any
+    instruction.
+
+    The singleton `InstructionContext` class serves as the predicate context
+    for these predicates.
+    """
+
+    def __init__(self):
+        # type: () -> None
+        self.name = 'inst'
+
+
+# Singleton instance.
+instruction_context = InstructionContext()
+
+
 class InstructionFormat(object):
     """
     Every instruction opcode has a corresponding instruction format which
@@ -48,6 +71,7 @@ class InstructionFormat(object):
     def __init__(self, *kinds, **kwargs):
         # type: (*Union[OperandKind, Tuple[str, OperandKind]], **Any) -> None # noqa
         self.name = kwargs.get('name', None)  # type: str
+        self.parent = instruction_context
 
         # The number of value operands stored in the format, or `None` when
         # `has_value_list` is set.