Fixed a bug in AaWriter.cpp.  The operand for a call was getting
incorrectly generated when it was to be the pointer to a global
variable. Added new function prepare_operand(Value*, Type*).
Fixed bug in FloatOperatorPackage.  TODO: need to specify
round style when calling the conversion functions.
For float->int conversion, changed the rounding style
to truncate (in order to be consistent with C behaviour).
Fixed bug:  when a constant was being returned, the return assignment
was being missed.