[llvm] [LoopFlatten] Invalidate SCEV on unflattening bail (PR #211819)

[Arda Serdar Pektezol via llvm-commits]

pektezol wrote:

@fhahn thank you for the reviews.
- I switched vectorizer to print SCEV. Therefore, I used update analyze checks to check the output.
- Added `REQUIRES: x86-registered-target`.
-  I had AI assistance to reduce more without the vectorizer (pretty much removing the for loop), and for name simplification.
- Tested with patched and unpatched opts from the same branch to confirm testcase fails on unpatched and passes on the fixed one.

https://github.com/llvm/llvm-project/pull/211819