[llvm] [Matrix] Add -debug-only prints when matrices get flattened (PR #142078)

[Florian Hahn via llvm-commits]

https://github.com/fhahn approved this pull request.

It would probably be good to add a test for the debug output?

Separately, it would probably also be good to add a STATISTIC for flattening, so we can easilt collect statistics and spot instances to investigate

https://github.com/llvm/llvm-project/pull/142078