[Ada] Fix expansion of attribute Loop_Entry wrt value propagation

[Ada] Fix expansion of attribute Loop_Entry wrt value propagation

When expanding attribute Loop_Entry we create constant object
declarations and put them just before the loop. The current values of
variables at the point of Loop_Entry attribute must not be used when
analysing the initialization expressions of these constants, because
they might be different from the values at the loop entry itself.

gcc/ada/

	* exp_attr.adb (Expand_Loop_Entry_Attribute): Disable value
	propagation when analysing the constant that holds the
	Loop_Entry prefix value.