[rtl] Assert that dummy instructions only write R0
[rtl] Assert that dummy instructions only write R0

Signed-off-by: Andreas Kurth <62c4a7f75976b7a2fb8d79afddaccc3e9236aad5@lowrisc.org>

lowRISC/ibex,lowRISC/ibex,lowRISC/ibex,lowRISC/ibex