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

[Min-Yih Hsu via llvm-commits]

mshockwave wrote:

> I think the cl::opt option is mainly useful for use with opt rather than clang. (Ideally we'd not use cl::opt for this and instead allow specifying pipeline tuning options from the command line...)

+1 on this, there are many many cases where we only want to run `opt` on an IR, either Clang takes too long or it's a reduced test case.  And it will be really helpful if we could have a command line flag to turn it off. Could we move `EnableLoopInterchange` to `tools/opt/NewPMDriver.cpp` and use it to populate PipelineTuningOptions instead of removing it completely?

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