[Ada] Avoid touching potentially nonexistent memory

[Pierre-Marie de Rodat]

...in cases where the Val_2 might cross a page boundary, and the second
page is now known to exist.

Copy_Bitfield is still disabled in the compiler: no test possible.

Tested on x86_64-pc-linux-gnu, committed on trunk

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

gcc/ada/

	* libgnat/s-bituti.adb (Get_Val_2, Set_Val_2): Use new routines
	for getting and setting a Val_2, avoiding touching the second
	half when that half might not exist.
	* exp_ch5.adb (Expand_Assign_Array_Loop_Or_Bitfield): Correct
	tests for potential volatile or independent components. In
	particular, do not call Prefix unless we know it's a slice.
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