ada: Improve expansion of nested conditional expressions in return statements

ada: Improve expansion of nested conditional expressions in return statements

This arranges for nested conditional expressions in simple return statements
to have their expansion delayed until the returns are distributed into their
dependent expressions.  This comprises the case of the elsif part of an if
expression present in the source code.

This also distributes qualified expressions into the dependent expressions
of conditional expressions, although this seems to occur rarely in practice.

gcc/ada/ChangeLog:

	* exp_aggr.ads (Is_Delayed_Conditional_Expression): Move to...
	* exp_aggr.adb (Is_Delayed_Conditional_Expression): Move to...
	(Convert_To_Assignments): Use Delay_Conditional_Expressions_Between.
	* exp_ch3.adb (Expand_N_Object_Declaration): Reset the Analyzed flag
	by means of Unanalyze_Delayed_Conditional_Expression.
	* exp_ch4.adb (Expand_N_Case_Expression): Likewise.  Delay expanding
	the expression if it is in the context of a simple return statement.
	(Expand_N_If_Expression): Likewise.
	(Expand_N_Qualified_Expression): Fold identical operand.  Distribute
	the expression into an operand that is a conditional expression with
	expansion delayed.
	(Process_Transient_In_Expression): Also test the parent node for the
	presence of a simple return statement.
	* exp_ch6.adb (Expand_Ctrl_Function_Call): Test the unconditional
	parent node for the presence of a simple return statement.
	* exp_util.ads (Delayed Expansion): New description.
	(Delay_Conditional_Expressions_Between): New procedure.
	(Is_Delayed_Conditional_Expression): ...here.
	(Unanalyze_Delayed_Conditional_Expression): New procedure.
	(Unconditional_Parent): New function.
	* exp_util.adb (Find_Hook_Context): Take into account conditional
	statements coming from conditional expressions.
	(Within_Conditional_Expression): Likewise.
	(Delay_Conditional_Expressions_Between): New procedure.
	(Is_Delayed_Conditional_Expression): ...here.
	(Unanalyze_Delayed_Conditional_Expression): New procedure.
	(Unconditional_Parent): New function.
	* sinfo.ads (Expansion_Delayed): Adjust description.