[rtl] By default, forward incoming instruction in C decoder
[rtl] By default, forward incoming instruction in C decoder

This commit modifies the compressed decoder to forward the incoming
instruction to the output. It is marked as legal, unless:
1) the decoder cannot determine if the instruction is compressed (e.g.
   because of unknown selector bits), or
2) the instruction is compressed but
    a) it cannot be successfully decompressed (e.g. because of unknown
       selector bits), or
    b) it is indeed illegal.

In the case of 2b) the compressed decoder may output an illegal
decompressed instruction instead of the incoming instruction.

Signed-off-by: Pirmin Vogel <386a7fc8e7338ed7899b4d9883c0ebec5de81366@lowrisc.org>

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