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

This patch adds support for reduction of min-intrinsic for scalar
types. Min 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.