Assert on lval_register

Assert on lval_register

This patch adds asserts where the value's lval must be lval_register.
This triggers an error in frame_register_unwind because VALUE_REGNUM
is used but value's lval is not lval_register.

This also reveals a design issue in frame_register_unwind, that is
arguments addrp and realnump are mutually exclusive, we either use
addrp (for lval_memory), or use realnump (for lval_register).  This
can be done in a separate patch.

gdb:

2016-12-06  Yao Qi  <yao.qi@linaro.org>

	* frame.c (frame_register_unwind): Set *realnump if *lvalp is
	lval_register.
	* value.c (deprecated_value_next_frame_id_hack): Assert
	value->lval is lval_register.
	(deprecated_value_regnum_hack): Likewise.