[Ada] Simplify analysis of assignment statements

[Ada] Simplify analysis of assignment statements

gcc/ada/

	* sem_ch5.adb (Set_Assignment_Type): Combine calls to Ekind
	using membership test.
	(Should_Transform_BIP_Assignment): Replace assignment to a
	"Result" variable with simple return statements; avoid repeated
	calls to Unqual_Conv by declaring a local constant.