[llvm] [LoopInterchange] Enable it by default (PR #124911)

[Ryotaro Kasuga via llvm-commits]

kasuga-fj wrote:

> > Maybe we should modify here?
> > https://github.com/llvm/llvm-project/blob/80b79ce432bbe12701fd9fe495ff9feeb5e4b9ca/clang/lib/Frontend/CompilerInvocation.cpp#L1964-L1965
> 
> This patch needs rebasing, it's old, in the meantime, I've added support for `-floop-interchange` which is now interacting with this and causing me a bit of pain. I.e., we now have -floop-interchange (and -fno-loop-interchange) and `-mllvm -enable-loopinterchange` to enable/disable it, and I have to worry about the different combinations. My question is @nikic , @kasuga-fj , do we want to keep both options around? Or shall we only keep the user-facing -floop-interchange, and get rid of LLVM option -enable-interchange? That would simplify things.

I personally think it's okay to remove `EnableLoopInterchange` if it causes confusion. But I don't know if there is a general policy on deleting existing options. 

https://github.com/llvm/llvm-project/pull/124911