optrecord_json_writer::optinfo_to_json can in theory be called from any
optimization pass, but currently uses get_fnname_from_decl, which
is RTL-specific.

In that PR, Jakub suggested using either DECL_ASSEMBLER_NAME or the
"printable name" (via current_function_name).

This patch makes it use DECL_ASSEMBLER_NAME.

gcc/ChangeLog:
	PR middle-end/89725
	* optinfo-emit-json.cc (optrecord_json_writer::optinfo_to_json):
	Use DECL_ASSEMBLER_NAME rather than get_fnname_from_decl.

From-SVN: r269994