c++: Only do maybe_init_list_as_range optimization if !processing_template_decl ...

c++: Only do maybe_init_list_as_range optimization if !processing_template_decl [PR108047]

The last testcase in this patch ICEs, because
maybe_init_list_as_range -> maybe_init_list_as_array
calls maybe_constant_init in:
  /* Don't do this if the conversion would be constant.  */
  first = maybe_constant_init (first);
  if (TREE_CONSTANT (first))
    return NULL_TREE;
but maybe_constant_init shouldn't be called when processing_template_decl.
While we could replace that call with fold_non_dependent_init, my limited
understanding is that this is an optimization and even if we don't optimize
it when processing_template_decl, build_user_type_conversion_1 will be
called again during instantiation with !processing_template_decl if it is
every instantiated and we can do the optimization only then.

2023-01-09  Jakub Jelinek  <jakub@redhat.com>

	PR c++/105838
	PR c++/108047
	PR c++/108266
	* call.cc (maybe_init_list_as_range): Always return NULL_TREE if
	processing_template_decl.

	* g++.dg/tree-ssa/initlist-opt2.C: New test.
	* g++.dg/tree-ssa/initlist-opt3.C: New test.