[llvm] [RISCV][GISEL] Legalization, register bank selection, and instruction selection for scalable G_SELECT (PR #85540)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> In that case, I made a mistake in my description. I wonder if we even need a separate opcode and we can just reuse G_SELECT as ISD::SELECT union ISD::VSELECT.
>

Yes. It never made sense to me why the DAG had a separate opcode, always seemed like pointless deviation from how IR works (same with fcmp/icmp->setcc) 

https://github.com/llvm/llvm-project/pull/85540