[flang][OpenMP] Move rewriting of min/max from Lower to Semantics (#153038)
There semantic analysis of the ATOMIC construct will require additional
rewriting (reassociation of certain expressions for user convenience),
and that will be driven by diagnoses made in the semantic checks.
While the rewriting of min/max is not required to be done in semantic
analysis, moving it there will make all rewriting for ATOMIC construct
be located in a single location.