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

[Michael Maitland via llvm-commits]

michaelmaitland wrote:

> Yes the only scalable shufflevector supported is a splat of element 0. The difference between ISD::SHUFFLE_VECTOR and ISD::SPLAT_VECTOR is that ISD::SHUFFLE_VECTOR takes a vector input and ISD::SPLAT_VECTOR takes a scalar input.

Do you think it would be reasonable to deal with converting a G_SHUFFLE_VECTOR into a G_SPLAT_VECTOR during legalization? Or are you asking for it to occur here during IRTranslation? FYI there is no G_SPLAT_VECTOR opcode today -- we would need to add that.

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