Code cleanup related to handling of attribute 'Old in Contract_Cases;
semantics is unaffected.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* exp_ch4.adb (Expand_N_Op_Eq): Reuse Is_Attribute_Result.
	* exp_prag.adb (Expand_Attributes): Reuse Is_Attribute_Old.