[Flang] Correct initial limit value in float min/maxloc reductions.

I was looking through to check whether Nan was being handled correctly, and
couldn't work out why simple cases were behaving differently. It turns out the
initial limit values was backwards for minloc/maxloc reductions. This fixes
that from llvm#79469.

Author: davemgreen

flang/lib/Optimizer/HLFIR/Transforms/OptimizedBufferization.cpp

@@ -854,7 +854,7 @@ class MinMaxlocElementalConversion : public mlir::OpRewritePattern<Op> {
         const llvm::fltSemantics &sem = ty.getFloatSemantics();
         return builder.createRealConstant(
             loc, elementType,
-            llvm::APFloat::getLargest(sem, /*Negative=*/!isMax));
+            llvm::APFloat::getLargest(sem, /*Negative=*/isMax));
       }
       unsigned bits = elementType.getIntOrFloatBitWidth();
       int64_t limitInt =

flang/test/HLFIR/maxloc-elemental.fir

@@ -110,6 +110,7 @@ func.func @_QPtest_float(%arg0: !fir.box<!fir.array<?xf32>> {fir.bindc_name = "a
   return
 }
 // CHECK-LABEL: _QPtest_float
+// CHECK:        %cst = arith.constant -3.40282347E+38 : f32
 // CHECK:        %[[V11:.*]] = fir.do_loop %arg3 = %c0 to %[[V10:.*]] step %c1 iter_args(%arg4 = %cst) -> (f32) {
 // CHECK-NEXT:     %[[V14:.*]] = arith.addi %arg3, %c1 : index
 // CHECK-NEXT:     %[[V15:.*]] = hlfir.designate %[[V1:.*]]#0 (%[[V14]])  : (!fir.box<!fir.array<?xf32>>, index) -> !fir.ref<f32>

flang/test/HLFIR/minloc-elemental.fir

@@ -295,6 +295,7 @@ func.func @_QPtest_float(%arg0: !fir.box<!fir.array<?xf32>> {fir.bindc_name = "a
   return
 }
 // CHECK-LABEL: _QPtest_float
+// CHECK:        %cst = arith.constant 3.40282347E+38 : f32
 // CHECK:        %[[V11:.*]] = fir.do_loop %arg3 = %c0 to %[[V10:.*]] step %c1 iter_args(%arg4 = %cst) -> (f32) {
 // CHECK-NEXT:     %[[V14:.*]] = arith.addi %arg3, %c1 : index
 // CHECK-NEXT:     %[[V15:.*]] = hlfir.designate %[[V1:.*]]#0 (%[[V14]])  : (!fir.box<!fir.array<?xf32>>, index) -> !fir.ref<f32>