ada: Do not unnecessarily use component-wise loop for slice assignment

This relaxes the condition under which Expand_Assign_Array leaves the
assignment to or from an array slice untouched.  The main prerequisite
for the code generator is that everything be aligned on byte boundaries
and Is_Possibly_Unaligned_Slice is too strong a predicate for this, so
it is replaced by the combination of Possible_Bit_Aligned_Component and
Is_Bit_Packed_Array, modulo a change to Possible_Bit_Aligned_Component
to take into account the specific case of slices.

gcc/ada/

	* exp_ch5.adb (Expand_Assign_Array): Adjust comment above the
	calls to Possible_Bit_Aligned_Component on the LHS and RHS. Do not
	call Is_Possibly_Unaligned_Slice in the slice case.
	* exp_util.ads (Component_May_Be_Bit_Aligned): Add For_Slice
	boolean parameter.
	(Possible_Bit_Aligned_Component): Likewise.
	* exp_util.adb (Component_May_Be_Bit_Aligned): Do not return False
	for the slice of a small record or bit-packed array component.
	(Possible_Bit_Aligned_Component): Pass For_Slice in recursive
	calls, except in the slice case where True is passed, as well as
	in call to Component_May_Be_Bit_Aligned.