Change register_changed_p returns bool

Change register_changed_p returns bool

register_changed_p actually returns bool, but return type is still int.
This patch changes the return type to bool.  The caller of
register_changed_p also checked whether the return value can be negative,
which is not needed now.  Such check was added in fb40c2090 in 2000,
at that moment, register_changed_p returns -1 when
read_relative_register_raw_bytes fails.  I can tell from its name that
it reads register contents, but we don't have this function called inside
register_changed_p, and the regcache is read-only.

gdb:

2017-11-24  Yao Qi  <yao.qi@linaro.org>

	* mi/mi-main.c (mi_cmd_data_list_changed_registers): Remove
	local 'changed'.  Remove error.
	(register_changed_p): Change return type to bool.