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

[Thorsten Schütt via llvm-commits]

tschuett wrote:

G_SELECT is defined as:
```
// Generic select
def G_SELECT : GenericInstruction {
  let OutOperandList = (outs type0:$dst);
  let InOperandList = (ins type1:$tst, type0:$src1, type0:$src2);
  let hasSideEffects = false;
}
```
It picks one of the two inputs. It is up to you to specify the relation between $type1 and $type0.

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