[flang] Control SUM simplification with a pass option.

The current code may not always work correctly, e.g.:
https://github.com/llvm/llvm-project/issues/57201

I added 'enable-experimental' pass option so that SUM simplification
may be enabled in LIT tests, but it is not enabled when the pass
is added to the passes pipeline.

Differential Revision: https://reviews.llvm.org/D131640