isaspec: Add "displayname" for altering {NAME} when decoding

isaspec: Add "displayname" for altering {NAME} when decoding

In afuc, we have the situation where there are a number of ALU
instructions with two (almost) completely different encodings, including
a different opcode location, etc. These need to be different leaf
bitsets with different names for the encoder to work, because otherwise
the encoder has no way of descriminating between them, but when
displaying them we want to use the same name. This adds a small facility
to make the name used for {NAME} when displaying and for the opcode
when encoding different, so that e.g. OPC_ADDI can display as "add"
instead of "addi".

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23949>