[clang] [flang] [llvm] add -floop-fuse to clang and flang (PR #142686)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

If the goal here is gcc compatibility, I'd suggest not hooking up the flag to anything; the existing LoopFusePass isn't used by anything and hasn't been touched in years, so it's very likely to have issues.

https://github.com/llvm/llvm-project/pull/142686