Introduce expression::first_opcode

Introduce expression::first_opcode

This adds a new helper method, expression::first_opcode, that extracts
the outermost opcode of an expression.  This simplifies some patches
in the expression rewrite series.

Note that this patch requires the earlier patch to avoid manual
dissection of OP_TYPE operations.

2020-12-15  Tom Tromey  <tom@tromey.com>

	* varobj.c (varobj_create): Use first_opcode.
	* value.c (init_if_undefined_command): Use first_opcode.
	* typeprint.c (whatis_exp): Use first_opcode.
	* tracepoint.c (validate_actionline): Use first_opcode.
	(encode_actions_1): Use first_opcode.
	* stack.c (return_command): Use first_opcode.
	* expression.h (struct expression) <first_opcode>: New method.
	* eval.c (parse_and_eval_type): Use first_opcode.
	* dtrace-probe.c (dtrace_process_dof_probe): Use first_opcode.