Avoid creating useless debug temporaries

insert_debug_temp_for_var_def has some strange code whereby it creates
debug temporaries for SINGLE_RHS (RHS for gimple_assign_single_p) but
not for other RHS in the same situation.

gcc/
	* tree-ssa.cc (insert_debug_temp_for_var_def): Do not create
	superfluous debug temporaries for single GIMPLE assignments.