[flang] Load fir.ref<fir.class<T>> instead of creating a wrong box

[clementval]

Summary

When a subroutine has an entry statement, the non-used argument
will be a fir.alloca and result in a fir.ref<fir.class<T>> for
polymorphic entities. In createBox, just load the box instead of
creating a wrong box.

[PeteSteinfeld]

All builds and tests correctly and looks good.

flang/lib/Optimizer/Builder/FIRBuilder.cpp

mlir::Value fir::FirOpBuilder::createBox(mlir::Location loc,
   if (itemAddr.getType().isa<fir::BaseBoxType>())
     return itemAddr;
   auto elementType = fir::dyn_cast_ptrEleTy(itemAddr.getType());
   if (!elementType) {
     mlir::emitError(loc, "internal: expected a memory reference type ")
         << itemAddr.getType();
     llvm_unreachable("not a memory reference type");
   }
+  if (itemAddr.getType().isa<fir::ReferenceType>() &&
+      elementType.isa<fir::BaseBoxType>())
+    return create<fir::LoadOp>(loc, fir::getBase(exv));
   mlir::Type boxTy = fir::BoxType::get(elementType);
   mlir::Value tdesc;
   if (isPolymorphic) {
     elementType = fir::updateTypeForUnlimitedPolymorphic(elementType);
     boxTy = fir::ClassType::get(elementType);
   }
 
   return exv.match(

flang/test/Lower/polymorphic.f90

 ! CHECK-SAME: %[[ARG0:.*]]: !fir.ref<!fir.box<!fir.heap<!fir.type<_QMpolymorphic_testTp2{a:i32,b:i32,c:f32}>>>> {fir.bindc_name = "p"}) {
 ! CHECK: %[[LOAD_ARG0:.*]] = fir.load %[[ARG0]] : !fir.ref<!fir.box<!fir.heap<!fir.type<_QMpolymorphic_testTp2{a:i32,b:i32,c:f32}>>>>
 ! CHECK: %[[RES:.*]] = fir.if %{{.*}} -> (!fir.box<!fir.type<_QMpolymorphic_testTp1{a:i32,b:i32}>>) {
 ! CHECK:   %[[REBOX:.*]] = fir.rebox %[[LOAD_ARG0:.*]] : (!fir.box<!fir.heap<!fir.type<_QMpolymorphic_testTp2{a:i32,b:i32,c:f32}>>>) -> !fir.box<!fir.type<_QMpolymorphic_testTp1{a:i32,b:i32}>>
 ! CHECK:   fir.result %[[REBOX]] : !fir.box<!fir.type<_QMpolymorphic_testTp1{a:i32,b:i32}>>
 ! CHECK: %[[CONV:.*]] = fir.convert %[[RES]] : (!fir.box<!fir.type<_QMpolymorphic_testTp1{a:i32,b:i32}>>) -> !fir.class<!fir.type<_QMpolymorphic_testTp1{a:i32,b:i32}>>
 ! CHECK: fir.call @_QMpolymorphic_testPtakes_p1_opt(%[[CONV]]) {{.*}} : (!fir.class<!fir.type<_QMpolymorphic_testTp1{a:i32,b:i32}>>) -> ()
 
+  subroutine class_with_entry(a)
+    class(p1) :: a,b
+    select type (a)
+    type is(p2)
+      print*, a%c
+    class default
+      print*, a%a
+    end select
+    return
+  entry d(b)
+    select type(b)
+    type is(p2)
+      print*,b%c
+    class default
+      print*,b%a
+    end select
+  end subroutine
+
+! CHECK-LABEL: func.func @_QMpolymorphic_testPclass_with_entry(
+! CHECK-SAME: %[[A:.*]]: !fir.class<!fir.type<_QMpolymorphic_testTp1{a:i32,b:i32}>> {fir.bindc_name = "a"}) {
+! CHECK: %[[B:.*]] = fir.alloca !fir.class<!fir.type<_QMpolymorphic_testTp1{a:i32,b:i32}>> {bindc_name = "b", uniq_name = "_QMpolymorphic_testFclass_with_entryEb"}
+
+! CHECK-LABEL: func.func @_QMpolymorphic_testPd(
+! CHECK-SAME: %[[B:.*]]: !fir.class<!fir.type<_QMpolymorphic_testTp1{a:i32,b:i32}>> {fir.bindc_name = "b"}) {
+! CHECK: %[[A:.*]] = fir.alloca !fir.class<!fir.type<_QMpolymorphic_testTp1{a:i32,b:i32}>> {bindc_name = "a", uniq_name = "_QMpolymorphic_testFclass_with_entryEa"}
+
 end module
 
 program test
   use polymorphic_test
   type(outer), allocatable :: o
   integer :: i(5)
   logical :: l(5)
   allocate(o)