rust: Use error_operand_p in rust-gcc.cc

rust: Use error_operand_p in rust-gcc.cc

Just a simple cleanupof the code to use error_operand_p
instead of directly comparing against error_mark_node.

This also moves some cdoe around when dealing with error_operand_p
just to be faster and/or slightly tighten up the code slightly.

gcc/rust/ChangeLog:

	* rust-gcc.cc (Bvariable::get_tree): Use error_operand_p.
	(pointer_type): Likewise.
	(reference_type): Likewise.
	(immutable_type): Likewise.
	(function_type): Likewise.
	(function_type_variadic): Likewise.
	Cleanup the check for receiver.type first.
	(function_ptr_type): Use error_operand_p.
	(fill_in_fields): Likewise.
	(fill_in_array): Likewise.
	(named_type): Likewise.
	(type_size): Likewise.
	(type_alignment): Likewise.
	(type_field_alignment): Likewise.
	(type_field_offset): Likewise.
	(zero_expression): Likewise.
	(float_constant_expression): Likewise.
	(convert_expression): Likewise.
	(struct_field_expression): Likewise.
	(compound_expression): Likewise.
	(conditional_expression): Likewise.
	(negation_expression): Likewise.
	(arithmetic_or_logical_expression): Likewise.
	(arithmetic_or_logical_expression_checked): Likewise.
	(comparison_expression): Likewise.
	(lazy_boolean_expression): Likewise.
	(constructor_expression): Likewise.
	(array_constructor_expression): Likewise.
	(array_index_expression): Likewise.
	(call_expression): Likewise.
	(init_statement): Likewise.
	(assignment_statement): Likewise.
	(return_statement): Likewise.
	(exception_handler_statement): Likewise.
	(if_statement): Likewise.
	(compound_statement): Likewise.
	Tighten up the code, removing t variable.
	(statement_list): Use error_operand_p.
	(block): Likewise.
	(block_add_statements): Likewise.
	(convert_tree): Likewise.
	(global_variable): Likewise.
	(global_variable_set_init): Likewise.
	(local_variable): Likewise.
	(parameter_variable): Likewise.
	(static_chain_variable): Likewise.
	(temporary_variable): Likewise.
	(function): Likewise. Tighten up the code.
	(function_defer_statement): Use error_operand_p.
	(function_set_parameters): Use error_operand_p.
	(write_global_definitions): Use error_operand_p.
	Tighten up the code around the loop.

Signed-off-by: Andrew Pinski <quic_apinski@quicinc.com>