[Flang][OpenMP] Fix crash in MAX intrinsic reduction

This patch fixes a crash that appear in an OpenMP MAX intrinsic
reduction when the reduction is not expressed using MAX intrinsic
function. Updated flang/test/Lower/OpenMP/wsloop-reduction-max.f90
test case with additional way of expressing MAX reduction.

Reviewed By: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D146967