[llvm] [RISCV] Support zext.h mnemonic with Zbkb. (PR #96821)

[Jessica Clarke via llvm-commits]

jrtc27 wrote:

Hm, I’m not sold on the assembly vs disassembly difference. Do we really need that weirdness, or can we limit that kind of thing to only be for real pseudoinstructions? 

https://github.com/llvm/llvm-project/pull/96821