Fix Todo : concern about illegal insn for dret call in non debug mode

Created by: silabs-PaulZ

Fixes one TODO in the RTL based on issue #430 (closed)

Concern was raised in a TODO comment if the core is in XRET_JUMP state and dret is called should result in an illegal exception if not in debug mode. Previous state, FLUSH_EX and FLUSH_WB will ensure dret call while not in debug_mode will invoke an illegal exception, preventing XRET_JUMP state and dret and !debug_mode.

Proven with temporary modification to debug_test as well as temporary formal check:

  a_xret_jump_dret_shall_be_debug_mode : assert property
  (
   @(posedge clk) (ctrl_fsm_cs==XRET_JUMP) && dret_dec_i |-> debug_mode_q
   ) ;

Signed-off-by: Paul Zavalney paul.zavalney@silabs.com