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

[Thorsten Schütt via llvm-commits]

tschuett 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

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