In the LIR, every register and variable must be defined before it is used. For method
parameters that are passed in fixed registers, exception objects passed to the exception
handler in a fixed register, or any other use of a fixed register not defined in this
method, an artificial definition is necessary. To avoid spill moves to be inserted
between the label at the beginning of a block an an actual definition in the second
instruction of a block, the registers are defined here in the label.