[llvm] [RISCV] Vectorize phi for loop carried @llvm.vp.reduce.* (PR #131974)

[Luke Lau via llvm-commits]

https://github.com/lukel97 approved this pull request.

LGTM, thanks

Can you update the comment above visitIntrinsicInst to say something like

```
// This affects ordered fadd reductions and VP reductions that have a scalar start value. // This tries to vectorize any scalar phis that feed into these reductions:
```

https://github.com/llvm/llvm-project/pull/131974