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

[Luke Lau via llvm-commits]

lukel97 wrote:

> LGTM.

Thanks. Will wait to hear back from @preames as well to make sure the correctness point is addressed

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