[llvm] [RISCV][GISEL] Add IRTranslation for shufflevector on scalable vector types (PR #80378)

[Thorsten Schütt via llvm-commits]

tschuett wrote:

@aemerson We would still need G_FSPLAT_VECTOR and G_SPLAT_VECTOR. If there are constants or operations?!?

https://github.com/llvm/llvm-project/pull/80378