ada: Small improvements to expansion of conditional expressions

ada: Small improvements to expansion of conditional expressions

They comprise using a nonnull accesss type for the indirect expansion to
avoid useless checks, smplifying the expansion of if expressions whose
condition is known at compile time to avoid an N_Expression_With_Actions,
using the indirect expansion for them in the indefinite case too, which
makes the special case for an unconstrained array type obsolete.

No functional changes.

gcc/ada/ChangeLog:

	* exp_ch4.adb (Expand_N_Case_Expression): Remove obsolete comment
	about C code generation.  Do not create a useless target type if
	the parent statement is rewritten instead of the expression.  Use
	a nonnull accesss type for the expansion done for composite types.
	(Expand_N_If_Expression): Simplify the expansion when the condition
	is known at compile time.  Apply the expansion done for by-reference
	types to indefinite types and remove the obsolete special case for
	unconstrained array types  Use a nonnull access type in this case.
	Rename New_If local variable to If_Stmt for the sake of consistency.