ada: Clean up slice-of-component optimization

ada: Clean up slice-of-component optimization

In the recursive case of Volatile_Or_Independent = False for array
types, fall through into later checks, so for example we check
the type of the prefix of a slice. The pattern here is "return True
in certain cases, otherwise fall through into the final 'return False'".

Remove check for "Ndim = 1"; if Slices is True, then the number of
dimensions is necessarily 1, because Ada doesn't have multi-dimensional
slices.

gcc/ada/

	* exp_ch5.adb
	(Expand_Assign_Array_Loop_Or_Bitfield): Minor cleanups.