c++: Fix up handling of vector CONSTRUCTORs with vectors in it in constexpr.cc [PR104226]

c++: Fix up handling of vector CONSTRUCTORs with vectors in it in constexpr.cc [PR104226]

The middle-end uses sometimes VECTOR_TYPE CONSTRUCTORs that contain
some other VECTOR_TYPE elements in it (should be with compatible element
size and smaller number of elements, e.g. a V8SImode vector can be
constructed as { V4SImode_var_1, V4SImode_var_2 }), and expansion of
__builtin_shufflevector emits these early, so constexpr.cc can see those
too.
constexpr.cc already has special cases for NULL index which is typical
for VECTOR_TYPE CONSTRUCTORs, and for VECTOR_TYPE CONSTRUCTORs that
contain just scalar elts that works just fine - init_subob_ctx just
returns on non-aggregate elts and get_or_insert_ctor_field has
  if (TREE_CODE (type) == VECTOR_TYPE && index == NULL_TREE)
    {
      CONSTRUCTOR_APPEND_ELT (CONSTRUCTOR_ELTS (ctor), index, NULL_TREE);
      return &CONSTRUCTOR_ELTS (ctor)->last();
    }
handling for it.  But for the vector in vector case init_subob_ctx would
try to create a sub-CONSTRUCTOR and even didn't handle the NULL index
case well, so instead of creating the sub-CONSTRUCTOR after the elts already
in it overwrote the first one.  So
(V8SImode) { { 0, 0, 0, 0 }, { 0, 0, 0, 0 } }
became
(V8SImode) { 0, 0, 0, 0 }
The following patch fixes it by not forcing a sub-CONSTRUCTOR for this
vector in vector case.

2022-01-26  Jakub Jelinek  <jakub@redhat.com>

	PR c++/104226
	* constexpr.cc (init_subob_ctx): For vector ctors containing
	vector elements, ensure appending to the same ctor instead of
	creating another one.

	* g++.dg/cpp0x/constexpr-104226.C: New test.