Also write out register assignments in write_instruction.
Also write out register assignments in write_instruction.

The value locations appear after the encodings:

> [R#0c,%x2]              v0 = iadd vx0, vx1
> [Iret#19]               return_reg v0

sunfishcode/cretonne,sunfishcode/cretonne,stoklund/cretonne,sunfishcode/cretonne,stoklund/cretonne,stoklund/cretonne