[llvm] [SimpleLoopUnswitch][NFC] move quadratic asserts under EXPENSIVE_CHECKS (PR #144887)

[Slava Zakharin via llvm-commits]

vzakhari wrote:

Looks good to me, though, I am not familiar with the code and cannot comment on @antoniofrighetto's concern.

https://github.com/llvm/llvm-project/pull/144887