[llvm] [CG][RISCV]Fix shuffling of odd number of input vectors (PR #125693)

[Alexey Bataev via llvm-commits]

alexey-bataev wrote:

> Not really sure the assert is worth the complexity here. I'd be tempted to just make the call to ExtractValue for Idx1 conditional on whether it's already in the set, and move on with life.

It is safe, generally speaking, I just wanted to add some extra checks here. I can remove all these checks

https://github.com/llvm/llvm-project/pull/125693