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

[Sebastian Pop via llvm-commits]

sebpop wrote:

The goal is to make it easier to turn on loop fusion and to collect and fix bug reports against loop fusion.  Once we are happy with the stability of the pass the goal is to enable by default in flang at -O2 and above.

GCC does not have an equivalent for -floop-fuse. 

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