Restrict alpha_convert_register_p

Restrict alpha_convert_register_p

This patch restricts alpha_convert_register_p from
"TYPE_LENGTH (type) != 8" to "TYPE_LENGTH (type) == 4", because,

 - we have check "TYPE_LENGTH (valtype) == 4" in alpha_register_to_value
   and alpha_value_to_register,
 - alpha lds and sts instruction access 4 bytes,
 - comments "It might need to convert the [float] register into the
   corresponding [integer] type (see Alpha)" and integer is 4-byte on
   alpha,

I think it is the right restrict condition to "TYPE_LENGTH (valtype) == 4".

gdb:

2017-05-24  Yao Qi  <yao.qi@linaro.org>

	* alpha-tdep.c (alpha_convert_register_p): Return true if type
	length is 4.
	(alpha_register_to_value): Remove type length check.
	(alpha_value_to_register): Likewise.