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

[Sebastian Pop via cfe-commits]

sebpop wrote:

> I'm no longer sure what's realistic

TBH, nobody writes such code: `a[j & 1][i & 1]`.
Anyway, we have a fix for all those.

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