New method regcache::assert_regnum

New method regcache::assert_regnum

class regcache has some methods checking the range of register number,
this patch is to move it in a new method assert_regnum.

gdb:

2017-11-02  Yao Qi  <yao.qi@linaro.org>

	* regcache.c (regcache::assert_regnum): New method.
	(regcache::invalidate): Call assert_regnum.
	(regcache::raw_update): Likewise.
	(regcache::raw_write): Likewise.
	(regcache::raw_read_part): Likewise.
	(regcache::raw_write_part): Likewise.
	(regcache::raw_supply): Likewise.
	(regcache::raw_supply_integer): Likewise.
	(regcache::raw_supply_zeroed): Likewise.
	(regcache::raw_collect): Likewise.
	(regcache::raw_collect_integer): Likewise.
	* regcache.h (regcache::assert_regnum): Declare.