[flang][Alias Analysis] not all block arguments are dummy arguments (#92156)

Arguments to openmp regions should not be tagged as dummy arguments.
This is particularly unsafe because these openmp blocks will eventually
be inlined into the calling function, where they will trivially alias
with other values inside of the calling function.

This is probably a theoretical issue because the calls to openmp runtime
function calls would act as barriers, preventing optimizations that are
too aggressive. But a lot more thought would need to go into a bet like
that.

This came out of discussion on
https://github.com/llvm/llvm-project/pull/92036