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

[Michael Maitland via llvm-commits]

michaelmaitland wrote:

> If I read the MachineVerifier correctly, then `G_SELECT (s1), (vector-ty), (vector-ty)` is valid.
> 
> https://github.com/llvm/llvm-project/blob/c51095f51b770f0663b65dd8a651601fca15701e/llvm/lib/CodeGen/MachineVerifier.cpp#L1371

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

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