[llvm] [LV] run verifyFunction once (PR #91470)

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

After some more thought, I realized that there is quite a steep cost to this change that I didn't think about when authoring the patch: it will slow down the development of LoopVectorize, and contributors will be greatly inconvenienced by having to go through the entire function's output to find the breakage. It primarily benefits the buildbot, but I don't think it justifies this cost.

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