Change value_contents_eq return bool

Change value_contents_eq return bool

This patch changes value_contents_eq return type from int to bool.

gdb:

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

	* mi/mi-main.c (register_changed_p): Update.
	* value.c (value_contents_bits_eq): Change return type.
	(value_contents_eq): Likewise.
	* value.h: Update comments.