[llvm] [RISCV] Custom legalize vp.merge for mask vectors. (PR #120479)

[Philip Reames via llvm-commits]

https://github.com/preames approved this pull request.

LGTM

I suspect you could widen to the next legal type instead, but this is fine for a functional fix.

https://github.com/llvm/llvm-project/pull/120479