[llvm] [RISCV] Legalize shuffle index after splitting two argument shuffles (PR #79330)

[Craig Topper via llvm-commits]

topperc wrote:

The description here is misleading. I believe returning SDValue() here causes the shuffle to get expanded through memory. It will go through the Expand path in LegalizeDAG.

https://github.com/llvm/llvm-project/pull/79330