[llvm] [RISCV][GISEL] instruction-select for G_SPLAT_VECTOR (PR #111193)

[Thorsten Schütt via llvm-commits]

tschuett wrote:

Sure I trust you. But you build the G_CONSTANT just for:
```
int64_t CstVal = MI.getOperand(1).getCImm()->getSExtValue();
```


https://github.com/llvm/llvm-project/pull/111193