[llvm] [LoopFusion] Return early when no two loops are at the same depth. (PR #226590)

[Ehsan Amiri via llvm-commits]

amehsan wrote:

Some AI driven experiments shows that:

1- Loops are already simplified when loop fusion starts.
2- The code here still saves compile time by a reduction in the number of postdom tree constructions. (This is counted during the entire compilation, not just in loop fusion).

So there seem to be genuine compile time savings by this patch. I will confirm the stats manually before marking this to be "ready for review".

https://github.com/llvm/llvm-project/pull/226590