[llvm] [RISCV] Consistently name AVL operands as $vl. NFC (PR #182174)

[Craig Topper via llvm-commits]

topperc wrote:

> meta question: should we add some documentation for this (and friends like `$sew`) convention? In case people forgot to do it when adding new instructions.

Yes, but where?

https://github.com/llvm/llvm-project/pull/182174