[llvm] [RISCV] Initial ISel support for the experimental zacas extension (PR #67918)

[Jessica Clarke via llvm-commits]

jrtc27 wrote:

> Migrated from https://reviews.llvm.org/D158956.

Please don't. From the big red banner at the top of Phabricator that should be hard to miss:

<img width="827" alt="image" src="https://github.com/llvm/llvm-project/assets/816232/74cf10e9-a9c3-48bd-94a6-fe74f4e7824b">

https://github.com/llvm/llvm-project/pull/67918