[llvm] [LoopPeel] Peel to make Phis loop inductions (PR #121104)

[Ryotaro Kasuga via llvm-commits]

https://github.com/kasuga-fj commented:

Currently there are some cases where undesirable peeling happens. As I checked, there were roughly two major patterns:

- Failed to analysis cast operations, like `induction_undesirable_peel1` I added in test.
- Peeling is performed properly to make PHIs into IVs, but such a peeling doesn't seem to be profitable. 
  - The main purpose of such peelings are for vectorization, but the loop cannot be vectorized due to other reason, e.g., it has function calls.

I think it's reasonable to address these issues in other patches. So, for now, I want to control this feature with a flag. After they are resolved, I'd like to enable it by default.

https://github.com/llvm/llvm-project/pull/121104