[clang] [flang] [flang] Enable FP sum reassociation by default (PR #218746)

[Tarun Prabhu via cfe-commits]

https://github.com/tarunprabhu approved this pull request.

The PR itself LGTM. If we are ok with enabling this at all optimization levels, it's fine with me.

https://github.com/llvm/llvm-project/pull/218746