[llvm] [LV, VPlan] Check if plan is compatible to EVL transform (PR #92092)

[Shih-Po Hung via llvm-commits]

arcbbb wrote:

> I found some loops can't be vectorized and finally I dug to this PR. :-) I simply commented out these lines in this PR and the loop can be vectorized correctly. What's the status now? Are there any things I missed?

It depends on the two PRs
- https://github.com/llvm/llvm-project/pull/118638
- https://github.com/llvm/llvm-project/pull/119859

Enabling the vectorization causes issues on HW that implements `ceil(AVL / 2) ≤ vl ≤ VLMAX if AVL < (2 * VLMAX)`
described in https://github.com/riscvarchive/riscv-v-spec/blob/master/v-spec.adoc#sec-vector-config
```
6.3 Constraints on Setting vl
The vset{i}vl{i} instructions first set VLMAX according to their vtype argument, then set vl obeying the following constraints:

vl = AVL if AVL ≤ VLMAX

ceil(AVL / 2) ≤ vl ≤ VLMAX if AVL < (2 * VLMAX)

vl = VLMAX if AVL ≥ (2 * VLMAX)
```

https://github.com/llvm/llvm-project/pull/92092