Add RISC-V encodings for spill and fill.

Add a Stack() class for specifying operand constraints for values on the
stack.

Add encoding recipes for RISC-V spill and fill instructions. Don't
implement the encoding recipe functions yet since we don't have the
stack slot layout yet.

lib/cretonne/meta/cdsl/registers.py

@@ -340,3 +340,15 @@ class Register(object):
         # type: (RegClass, int) -> None
         self.regclass = rc
         self.unit = unit
+
+
+class Stack(object):
+    """
+    An operand that must be in a stack slot.
+
+    A `Stack` object can be used to indicate an operand constraint for a value
+    operand that must live in a stack slot.
+    """
+    def __init__(self, rc):
+        # type: (RegClass) -> None
+        self.regclass = rc