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

[Michael Maitland via llvm-commits]

michaelmaitland wrote:

> GlobalIsel has no splat_vector yet. I was hoping that #74502 would make progress.

We might still want a G_SPLAT_VECTOR in the case that the splat is not constant. WDYT?

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