[rtl] Move some assertions around
[rtl] Move some assertions around

- Prevents errors due to empty generate blocks when passed through sv2v

Signed-off-by: Tom Roberts <f3b1a53ee1cbeb35a47033dea932030093853292@lowrisc.org>

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