[clang] [llvm] [RISCV] Make zvknha a subset of zvknhb (PR #178680)

[Nicolas Brunie via cfe-commits]

nibrunieAtSi5 wrote:

> zvknha and zvknhb are mutually exclusive [riscv/riscv-crypto#364 (comment)](https://github.com/riscv/riscv-crypto/issues/364#issuecomment-1726782096)

<img width="947" height="249" alt="image" src="https://github.com/user-attachments/assets/996a3c0b-2812-4f0f-af4c-d40d042c043b" />


I don't know why that guy is saying that.
More seriously, I think this answer was a mistake from my side (it was aligned with the way it was defined at the time in the TG, but this does not align with the way it is done on the other RISC-V spec).
I think we should open an issue on **riscv-isa-manual** (if it is not already done) and discuss clarification of that point there.

https://github.com/llvm/llvm-project/pull/178680