Change InstBuilder low-level format constructor signatures.

The per-instruction format low-level constructors in InstBuilder should
be independent of the relative ordering of value and immediate operands
in order to prepare for the future instruction format merger.

Reorder their arguments such that all the immediate operands are placed
before the value operands.

For instruction formats that use a value list representation, just take
a single ValueList argument. The value lists are created by the
individual instruction constructors. This means that the format
constructor doesn't care how many of the instructions operands are
'fixed' and how many are 'variable' arguments.

lib/cretonne/meta/cdsl/operands.py

@@ -158,6 +158,13 @@ class Operand(object):
         """
         return self.kind is VALUE
 
+    def is_varargs(self):
+        # type: () -> bool
+        """
+        Is this a VARIABLE_ARGS operand?
+        """
+        return self.kind is VARIABLE_ARGS
+
     def is_immediate(self):
         # type: () -> bool
         """