[llvm] [RISCV] Allow folding vmerge with implicit passthru when true has tied dest (PR #78565)

[Luke Lau via llvm-commits]

lukel97 wrote:

> Everywhere you wrote "implicit" did you meant "implicit_def"?

Yes, but I think nowadays this might actually be NoRegister

https://github.com/llvm/llvm-project/pull/78565