[llvm] [RISCV] Select Zvkb VANDN for shorter constant loading sequences (PR #123345)

[Piotr Fusik via llvm-commits]

pfusik wrote:

I'm trying to extend #120221 to VANDN.

Reproducer:
```c
void mask(short *a) {
    for (int i = 0; i < 256; i++)
        a[i] &= 0x7fff;
}
```

I understand this will also need an update in `RISCVDAGToDAGISel::selectInvLogicImm` to allow splat vector uses, but gdb shows it doesn't even enter this function, so my TableGen pattern must be wrong, but I don't know why. Please help.

https://github.com/llvm/llvm-project/pull/123345