Add support for tied operand constraints.

The register constraint for an output operand can be specified as an
integer indicating the input operand number to tie. The tied operands
must use the same register.

Generate operand constraints using ConstraintKind::Tied(n) for both the
tied operands. The n index refers to the opposite array. The input
operand refers to the outs array and vice versa.

lib/cretonne/meta/cdsl/isa.py

@@ -7,7 +7,7 @@ from .ast import Apply
 # The typing module is only required by mypy, and we don't use these imports
 # outside type comments.
 try:
-    from typing import Tuple, Union, Any, Iterable, Sequence, List, Set, TYPE_CHECKING  # noqa
+    from typing import Tuple, Union, Any, Iterable, Sequence, List, Set, Dict, TYPE_CHECKING  # noqa
     if TYPE_CHECKING:
         from .instructions import MaybeBoundInst, InstructionGroup, InstructionFormat  # noqa
         from .predicates import PredNode  # noqa
@@ -220,13 +220,27 @@ class EncRecipe(object):
             if isinstance(c, int):
                 # An integer constraint is bound to a value operand.
                 # Check that it is in range.
-                assert c >= 0
-                if not self.format.has_value_list:
-                    assert c < self.format.num_value_operands
+                assert c >= 0 and c < len(self.ins)
             else:
                 assert isinstance(c, RegClass) or isinstance(c, Register)
         return seq
 
+    def ties(self):
+        # type: () -> Tuple[Dict[int, int], Dict[int, int]]
+        """
+        Return two dictionaries representing the tied operands.
+
+        The first maps input number to tied output number, the second maps
+        output number to tied input number.
+        """
+        i2o = dict()  # type: Dict[int, int]
+        o2i = dict()  # type: Dict[int, int]
+        for o, i in enumerate(self.outs):
+            if isinstance(i, int):
+                i2o[i] = o
+                o2i[o] = i
+        return (i2o, o2i)
+
 
 class Encoding(object):
     """