[Ada] Optimize nonstandard boolean conversions

This patch improves the generated code for nonstandard boolean types.

One of the improvements extends the code that avoids converting back
to the nonstandard boolean type an expression computed as standard
boolean, when it will be converted to a(nother) nonstandard boolean
type.

The other improvement involves using the literal representation
constants in an If_Expression instead of dereferencing the T'Val array
when converting to a (nonstandard) boolean type. Avoiding the array
dereference enables the compiler middle-end to propagate the constants
and perform optimizations based on them, to the point of obviating the
improvement above.

Unfortunately, the code generated with this alternate expansion tends
to be slightly larger if it turns out to not enable any further
optimization, though it's most certainly faster, especially on targets
with conditional moves, more so if "store flag" is slow, as on x86.
Still, the array dereference is more straightforward and shorter, so
I've arranged for this alternate expansion to be used only when
optimizing for speed.

gcc/ada/

	* exp_util.adb (Adjust_Result_Type): Leave result in
	Standard.Boolean if it's going to be converted to another
	boolean type.
	* exp_ch4.adb (Expand_N_Type_Conversion): When optimizing,
	convert to nonstandard booleans with an if_expression with
	boolean literals.

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