This is a follow-on to https://reviews.llvm.org/D134073.
It renames a few fields to have consistent names, as well as renaming
operands to match the field names.
The encoder behavior is unchanged by this cleanup, but a few
instructions were previously being disassembled incorrectly, and have
been corrected by this change. All of the affected instructions were
missing disassembly tests, which are now added.
This can use INST-COUNT-8: