[llvm] [LoopInterchange] Don't consider loops with BTC=0 (PR #167113)

[Sjoerd Meijer via llvm-commits]

sjoerdmeijer wrote:

The last update address @kasuga-fj 's remark:
- It moves the code from `isComputableLoopNest` to avoid that a whole loop nest is rejected, and
- a new test has been added to check that ,
- The checks now live in `isProfitable`,
- A bit of code has been added to avoid recalculating the BTC; the BTC is calculated early on, and that is now passed on to the later stage. 
- Some existing tests needed patching up because they still had a BTC=0, I missed that in the clean up patch https://github.com/llvm/llvm-project/pull/167748. 


https://github.com/llvm/llvm-project/pull/167113