[clang] [flang] [flang] add -floop-interchange and enable it with opt levels (PR #140182)

[Sebastian Pop via cfe-commits]

sebpop wrote:

> Do you have any compilation time and performance data?

@madhur13490 did several changes to loop interchange to optimize the overall compilation time with the pass.  I believe Madhur has only looked at c/c++ benchmarks and not at how loop interchange would impact flang. I think that if compilation time is good for c/c++, it should also be good for fortran.

On the perf side, I was looking if we can already catch swim from cpu2000, and that fails with not enough data to infer number of iterations.  I will be working on adding assume (N < 1335) based on analyzing array decls and infer loop bounds.

https://github.com/llvm/llvm-project/pull/140182