Fix verbose print of instrs that omit low imm bits

[elliotb-lowrisc]

Modify unscatter so it works for instructions that do not encode the lowest bits of the immediate value provided to them. This issue has been hidden because many instructions have been encoded using the lower immediate bits rather than upper immediate bits (e.g. imm[19:0] rather than imm[31:12] for lui), and others (which do omit lower immediate bits) are not included in disassembly output (e.g. c_lui with empty rv_c_disass).

See also:
#41

[elliotb-lowrisc]

I discovered this issue when testing the CHERIoT auicgp instruction, which I defined in RV32_Xcheri.hs as:

auicgp_raw                         =                                        "imm[31:12] cd[4:0] 1111011"
auicgp cd imm                      = encode auicgp_raw                       imm       cd

rv32_xcheri_disass = [ ...
                     , auicgp_raw                      --> prettyU "auicgp"

[marnovandermaas]

This looks reasonable to me, but I will leave it to someone with more Haskell knowledge to approve this.

[gameboo]

I think this makes sense, thanks for the PR. I just want to ping @mn416 for a sanity check on that.

[mn416]

Looks fine to me, other than the apparent inconsistency w.r.t. lui raised by @elliotb-lowrisc. (Should auicgp and lui not be defined in the same style?)

[PeterRugg]

Happy to merge this: the inconsistency is a broader issue.