[Flang][OpenMP] Add support for OpenMP max reduction

This patch adds support for reduction of max-intrinsic for scalar
types. Max is lowered as a compare-select in the default lowering
flow for Flang. This pattern is matched and replaced with the
OpenMP dialect reduction operation.

Note: This is a temporary flow. The plan is to move to a flow where
the OpenMP reduction operation is inserted during lowering.

Reviewed By: do

Differential Revision: https://reviews.llvm.org/D145083