[llvm] [RISCV] Fix same mask vmerge peephole discarding false operand (PR #107827)

[Roger Ferrer Ibáñez via llvm-commits]

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

I've been testing this patch and fixes all the issues we encountered. Thanks @lukel97 !

https://github.com/llvm/llvm-project/pull/107827