[Ada] Fix expansion of operations on nonbinary modular types

[Ada] Fix expansion of operations on nonbinary modular types

2018-10-09  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* exp_ch4.adb (Expand_Modular_Op): When expanding an operation
	on nonbinary modular types, convert the opersnds to an integer
	type that is large enough to hold the modulus of the type, which
	may be larger than Integer'Last.

From-SVN: r264973