[Ada] Refine conditions for calling Copy_Bitfield

[Ada] Refine conditions for calling Copy_Bitfield

Avoid calling Copy_Bitfield if there are volatile or independent
components that might be read or written. The test is conservative.

2019-09-17  Bob Duff  <duff@adacore.com>

gcc/ada/

	* exp_ch5.adb (Expand_Assign_Array_Loop_Or_Bitfield): Add tests
	for potential volatile or independent components.
	* libgnat/s-bituti.adb (Copy_Small_Bitfield,
	Copy_Large_Bitfield): Move declarations to more appropriate
	place.

From-SVN: r275768