Generation of illegal instructions with corev-dv (riscv-dv)

In pull-request #2380 I commented that the function insert_illegal_instr() in cv32e40p/env/corev-dv/instr_lib/cv32e40p_float_instr_lib.sv does not actually create illegal instructions(!). I now understand why that is, but the next person to see this code will have the same question.

Please add a comment block explaining the rationale for this function and how corev-dv actually generates illegal instructions.