[otbn] Remove two extra cases from the InsnOpcodeBaseOp decoder
[otbn] Remove two extra cases from the InsnOpcodeBaseOp decoder

These correspond to the SLT and SLTU instructions (Set Less Than and
Set Less Than Unsigned), which we don't support.

Also, add comments to the remaining cases to show which instructions
they correspond to (which seems to be rather helpful when trying to
debug weird decoder issues).

Signed-off-by: Rupert Swarbrick <290093e2ad2515ed2f14d749293c8cf8e02ca274@lowrisc.org>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan