[llvm] [RISCV] Remove mfd and mpd CSR names for SiFive. (PR #75392)

[Garvit Gupta via llvm-commits]

quic-garvgupt wrote:

Hi @topperc 
This patch along with another patch(https://github.com/llvm/llvm-project/commit/6dc5ba4cca72a5c25597722b8a8c7dcff5fb67be 
) were introduced sometime back to add support for custom instructions and Control and Status Registers (CSRs) specific to S76 processo, were subsequently reverted by the community, citing concerns that these instructions and CSRs do not fall under the SiFive Custom Instruction Extension (SCIE).

We believe that documenting these custom instructions and CSRs is crucial because implementation of these custom instructions and CSRs involved significant changes in the backend framework. Specifically, handling encoding, resolving name conflicts, and creating a distinct vendor-specific “searchable-tables” were part of this effort. 

Since Sifive is willing to define these under a different extension, do let us know about the same!

CC: @apazos 

https://github.com/llvm/llvm-project/pull/75392