ada: Disable slice-of-component optimization in some cases

ada: Disable slice-of-component optimization in some cases

This patch disables the Fast_Copy_Bitfield optimization in certain rare
cases that currently do not work (crash in gigi). We could improve
Expand_Assign_Array_Bitfield_Fast to handle these cases properly, but
that change is delicate, so for now, we simply disable the optimization.

gcc/ada/

	* exp_ch5.adb
	(Expand_Assign_Array_Loop_Or_Bitfield): Disable the
	Fast_Copy_Bitfield optimization in certain cases.