Legalize unsigned-to-float conversions for Intel 64.

Also make sure we generate type checks for the controlling type variable
in legalization patterns. This is not needed for encodings since the
encoding tables are already keyed on the controlling type variable.

lib/cretonne/meta/cdsl/predicates.py

@@ -35,7 +35,8 @@ try:
         from .typevar import TypeVar  # noqa
         PredContext = Union[SettingGroup, InstructionFormat,
                             InstructionContext]
-        PredLeaf = Union[BoolSetting, 'FieldPredicate', 'TypePredicate']
+        PredLeaf = Union[BoolSetting, 'FieldPredicate', 'TypePredicate',
+                         'CtrlTypePredicate']
         PredNode = Union[PredLeaf, 'Predicate']
         # A predicate key is a (recursive) tuple of primitive types that
         # uniquely describes a predicate. It is used for interning.
@@ -373,3 +374,42 @@ class TypePredicate(object):
         """
         return 'dfg.value_type(args[{}]) == {}'.format(
                 self.value_arg, self.value_type.rust_name())
+
+
+class CtrlTypePredicate(object):
+    """
+    An instruction predicate that checks the controlling type variable
+
+    :param value_type: The required value type.
+    """
+
+    def __init__(self, value_type):
+        # type: (ValueType) -> None
+        assert value_type is not None
+        self.value_type = value_type
+
+    def __str__(self):
+        # type: () -> str
+        return 'ctrl_typevar:{}'.format(self.value_type)
+
+    def predicate_context(self):
+        # type: () -> PredContext
+        return instruction_context
+
+    def predicate_key(self):
+        # type: () -> PredKey
+        return ('ctrltypecheck', self.value_type.name)
+
+    def predicate_leafs(self, leafs):
+        # type: (Set[PredLeaf]) -> None
+        leafs.add(self)
+
+    def rust_predicate(self, prec):
+        # type: (int) -> str
+        """
+        Return Rust code for evaluating this predicate.
+
+        It is assumed that the context has `dfg` and `inst` variables.
+        """
+        return 'dfg.ctrl_typevar(inst) == {}'.format(
+                self.value_type.rust_name())