Generate a Builder data type. WIP.

The Builder provides a convenient interface for inserting instructions
into an extended basic block.

The bulk of the builder methods are generated automatically from the
meta language instruction descriptions.

Still TODO: Keep track of an insertion position.

meta/cretonne/base.py

@@ -1057,7 +1057,7 @@ fdemote = Instruction(
         ins=x, outs=a)
 
 x = Operand('x', Float)
-a = Operand('a', Int)
+a = Operand('a', IntTo)
 
 fcvt_to_uint = Instruction(
         'fcvt_to_uint', r"""
@@ -1083,6 +1083,9 @@ fcvt_to_sint = Instruction(
         """,
         ins=x, outs=a)
 
+x = Operand('x', Int)
+a = Operand('a', FloatTo)
+
 fcvt_from_uint = Instruction(
         'fcvt_from_uint', r"""
         Convert unsigned integer to floating point.