consider opcode bits placement for future instructions

[jnk0le]

as everything outside v0.70 got postponed, i think it's worth to note that some of the current compressed encodings may lead to awkward encoding of potential new instructions.

e.g. #158 there is potential encoding with immediate symmetrical to c.lui

but current load/stores occupy all combinations of bits [11:10] with bit 12 set to 0, which means that any new istructions have to fit next to c.sh or with bit 12 set to 1. 12th bit is also the only one used for sign extensions by other instructions.

[tariqkurd-repo]

I'll only change encodings if the architecture review committee require it (also they set the encodings anyway)

[jnk0le]

Moving these three extensions to the frozen state should now just be a formality.

maybe should poke @aswaterman before its too late again.