[llvm] [LoopInterchange] Constrain number of load/stores in a loop (PR #118973)

[Sjoerd Meijer via llvm-commits]

sjoerdmeijer wrote:

The idea makes sense, I think. I.e., bailing out before the more expensive dependence checks would be the sensible thing to do. But we do need numbers to discuss this trade-off:
- the compile time savings,
- the number of interchanges that we miss out on, if any. 

Maybe you can share those numbers for the LLVM test-suite here. Ideally, trigger the same amount of times, and save compile-time by doing the check upfront. 

Also, it is probably better to now turn that constant `MaxMemInstrCount` into an option with a default value, so that it could be easily adjusted on the command line for testing purposes.

https://github.com/llvm/llvm-project/pull/118973