[clang] [llvm] [RISCV] Assembler support for XRivosVizip (PR #127694)

[Philip Reames via cfe-commits]

preames wrote:

> LGTM

@topperc Could you double check to make sure this still holds?  I'd missed the register overlap constraint, so I added that plus testing both for that case, and the masked instruction case which I'd not previously had tests for.  

https://github.com/llvm/llvm-project/pull/127694