[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 #73101

Author: NimishMishra

flang/lib/Lower/OpenMP.cpp

@@ -754,7 +754,7 @@ static mlir::Value getReductionInitValue(mlir::Location loc, mlir::Type type,
     if (auto ty = type.dyn_cast<mlir::FloatType>()) {
       const llvm::fltSemantics &sem = ty.getFloatSemantics();
       return builder.createRealConstant(
-          loc, type, llvm::APFloat::getSmallest(sem, /*Negative=*/true));
+          loc, type, llvm::APFloat::getLargest(sem, /*Negative=*/false));
     }
     unsigned bits = type.getIntOrFloatBitWidth();
     int64_t maxInt = llvm::APInt::getSignedMaxValue(bits).getSExtValue();

flang/test/Lower/OpenMP/FIR/wsloop-reduction-min.f90

@@ -2,7 +2,7 @@
 ! RUN: %flang_fc1 -emit-fir -flang-deprecated-no-hlfir -fopenmp -o - %s 2>&1 | FileCheck %s
 
 !CHECK: omp.reduction.declare @[[MIN_DECLARE_F:.*]] : f32 init {
-!CHECK:   %[[MAXIMUM_VAL_F:.*]] = arith.constant -1.401300e-45 : f32
+!CHECK:   %[[MAXIMUM_VAL_F:.*]] = arith.constant 3.40282347E+38 : f32
 !CHECK:   omp.yield(%[[MAXIMUM_VAL_F]] : f32)
 !CHECK: combiner
 !CHECK: ^bb0(%[[ARG0_F:.*]]: f32, %[[ARG1_F:.*]]: f32):

flang/test/Lower/OpenMP/wsloop-reduction-min.f90

@@ -2,7 +2,7 @@
 ! RUN: %flang_fc1 -emit-hlfir -fopenmp -o - %s 2>&1 | FileCheck %s
 
 !CHECK: omp.reduction.declare @[[MIN_DECLARE_F:.*]] : f32 init {
-!CHECK:   %[[MAXIMUM_VAL_F:.*]] = arith.constant -1.401300e-45 : f32
+!CHECK:   %[[MAXIMUM_VAL_F:.*]] = arith.constant 3.40282347E+38 : f32
 !CHECK:   omp.yield(%[[MAXIMUM_VAL_F]] : f32)
 !CHECK: combiner
 !CHECK: ^bb0(%[[ARG0_F:.*]]: f32, %[[ARG1_F:.*]]: f32):