[llvm] [RISCV][VLOPT] Fix passthru check in getOperandInfo (PR #112244)

[Luke Lau via llvm-commits]

lukel97 wrote:

> Do we need to fix in isVectorOpUsedAsScalarOp too?

I think isVectorOpUsedAsScalarOp is actually correct currently, but because HasPassthru is always true. I don't think it's correct if HasPassthru is false but reductions always have a passthru, so it's never hit.

```c++
  case RISCV::VFWREDUSUM_VS: {
    bool HasPassthru = RISCVII::isFirstDefTiedToFirstUse(MI->getDesc());
    return HasPassthru ? MO.getOperandNo() == 2 : MO.getOperandNo() == 3;
```

https://github.com/llvm/llvm-project/pull/112244