[llvm] LoopVectorize: run verifyFunction once (PR #91470)

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

I managed to do some statistical benchmarking of this patch, and the results are good:

| Command | Mean [s] | Min [s] | Max [s] |
|:---|---:|---:|---:|
| main | 8.471 ± 0.012 | 8.457 | 8.495 |
| lv-verify-once | 8.462 ± 0.015 | 8.446 | 8.485 |

https://github.com/llvm/llvm-project/pull/91470