preparation for reversing StackDirection.

Fixed objects are created in the following order:  
 1. Incoming arguments passed on stack.
 2. va_arg objects (include both arguments that are passed in registers and
    pointer to the location of the first va_arg argument).
 3. $gp restore slot.
 4. Outgoing arguments passed on stack.
 5. Pointer to alloca'd space.

llvm-svn: 131767