[libre-riscv-dev] Instruction sorta-prefixes for easier high-register access

[Luke Kenneth Casson Leighton]

hi jacob, a brief thought / reply, more later:

* prefixes on instructions gives an opportunity to do key-hole
analysis, assessing if the prefixes can be removed and use the CSR
stack instead.
* thank you for explaining about predicate registers.
* one thought, have branch overwrite the src predicate.
* i like the VL multiplier idea.
* we don't want to go completely overboard, just the main uses.  CSR
stack can cover the "full" features
* am going to be relying on you to give feedback and insight on what
the main uses are!

l.