[flang] Lower evaluate::Extremum

Update BinaryOp<T>::gen so that const T& is threaded and some
operation knowledge that is not encoded by the type T or the arguments
can be used. For Extremum, it is the order (Greater or Lesser) that is
required, but this will also be required for evaluate::Relational.

Differential Revision: https://reviews.llvm.org/D139124