[flang][NFC] Add select type test to check correct ordering

[clementval]

Summary

Somehow this test has been left behind in my sandbox. This
patch adds a lowering test for fir.select_type operation and
makes sure the dynamic type comaparison is done in the right
order when we have multiple CLASS IS type guard statement
for types that are linked.

This should have been posted with D138280.

[PeteSteinfeld]

All builds and tests correctly and looks good.

flang/test/Lower/select-type-2.f90

+! RUN: bbc -polymorphic-type -emit-fir %s -o - | fir-opt --fir-polymorphic-op | FileCheck %s
+module select_type_2
+  type p1
+    integer :: a
+    integer :: b
+  end type
+
+  type, extends(p1) :: p2
+    integer :: c
+  end type
+
+  type, extends(p2) :: p3
+    integer :: d
+  end type
+
+contains
+
+  subroutine select_type1(a)
+    class(p1), intent(in) :: a
+
+    select type (a)
+    class is (p1)
+      print*, 'class is p1'
+    class is (p3)
+      print*, 'class is p3'
+    class default
+      print*,'default'
+    end select
+  end subroutine
+
+! CHECK-LABEL: func.func @_QMselect_type_2Pselect_type1(
+! CHECK-SAME: %[[ARG0:.*]]: !fir.class<!fir.type<_QMselect_type_2Tp1{a:i32,b:i32}>> {fir.bindc_name = "a"}) {
+! CHECK:      %[[TDESC_P3_ADDR:.*]] = fir.address_of(@_QMselect_type_2E.dt.p3) : !fir.ref<!fir.type<{{.*}}>>
+! CHECK:      %[[TDESC_P3_CONV:.*]] = fir.convert %[[TDESC_P3_ADDR]] : (!fir.ref<!fir.type<{{.*}}>>) -> !fir.ref<none>
+! CHECK:      %[[BOX_NONE:.*]] = fir.convert %[[ARG0]] : (!fir.class<!fir.type<_QMselect_type_2Tp1{a:i32,b:i32}>>) -> !fir.box<none>
+! CHECK:      %[[CLASS_IS_CMP:.*]] = fir.call @_FortranAClassIs(%[[BOX_NONE]], %[[TDESC_P3_CONV]]) : (!fir.box<none>, !fir.ref<none>) -> i1
+! CHECK:      cf.cond_br %[[CLASS_IS_CMP]], ^[[CLASS_IS_P3_BLK:.*]], ^[[NOT_CLASS_IS_P3_BLK:.*]]
+! CHECK:    ^bb[[NOT_CLASS_IS_P1:[0-9]]]:
+! CHECK:      cf.br ^bb[[DEFAULT_BLK:[0-9]]]
+! CHECK:    ^bb[[CLASS_IS_P1:[0-9]]]:
+! CHECK:      cf.br ^bb[[END_SELECT_BLK:[0-9]]]
+! CHECK:    ^[[NOT_CLASS_IS_P3_BLK]]:
+! CHECK:      %[[TDESC_P1_ADDR:.*]] = fir.address_of(@_QMselect_type_2E.dt.p1) : !fir.ref<!fir.type<{{.*}}>>
+! CHECK:      %[[TDESC_P1_CONV:.*]] = fir.convert %[[TDESC_P1_ADDR]] : (!fir.ref<!fir.type<{{.*}}>>) -> !fir.ref<none>
+! CHECK:      %[[BOX_NONE:.*]] = fir.convert %[[ARG0]] : (!fir.class<!fir.type<_QMselect_type_2Tp1{a:i32,b:i32}>>) -> !fir.box<none>
+! CHECK:      %[[CLASS_IS_CMP:.*]] = fir.call @_FortranAClassIs(%[[BOX_NONE]], %[[TDESC_P1_CONV]]) : (!fir.box<none>, !fir.ref<none>) -> i1
+! CHECK:      cf.cond_br %[[CLASS_IS_CMP]], ^bb[[CLASS_IS_P1]], ^bb[[NOT_CLASS_IS_P1]]
+! CHECK:    ^[[CLASS_IS_P3_BLK]]:
+! CHECK:      cf.br ^bb[[END_SELECT_BLK]]
+! CHECK:    ^bb[[DEFAULT_BLK]]:
+! CHECK:      cf.br ^bb[[END_SELECT_BLK]]
+! CHECK:    ^bb[[END_SELECT_BLK]]:
+! CHECK:      return
+
+  end module