[OpenMP][FIX] Remove unsound reasoning about written to values

[OpenMP][FIX] Remove unsound reasoning about written to values

Even if a value is for sure written we need to visit the call sites as
they might end up inside the function that reads and writes the value.
In a follow up we can introduce correct reasoning to avoid the backwards
traversal in this case and instead check if any call site between the
write and the read might reach a potential write we want to exclude.