[flang][OpenMP] Fix min reduction initialization (#73102)

Initialization of reduction variable for min-reduction is set to largest
negative value. As such, in presence of non-negative operands, min
reduction gives incorrect output. This patch initialises min-reduction
to use the maximum positive value instead, so that it can produce
correct output for the entire range of real valued operands.

Fixes https://github.com/llvm/llvm-project/issues/73101