[llvm] [RISCV] Add imply rules for zcd/zcf. (PR #91342)

[Alex Bradbury via llvm-commits]

asb wrote:

> I don’t think this is valid, Zc* are subsets of C but not technically compatible?

I think it is indeed correct, and follows what is described in the ISA manual:
https://github.com/riscv/riscv-isa-manual/blob/7b8ddc9f58b858265525b59f45bfca961f805b6b/src/zc.adoc#L6

We haven't gone and e.g. made M imply Zmmul though.

I think in general we haven't gone back and added these implications when new subset extensions are added as we're cautious of it causing issues with no benefit. We have of course aimed to add such implications when adding new extensions that have these relationships defined and weren't previously supported in LLVM.

https://github.com/llvm/llvm-project/pull/91342