[Ada] Optimize nonstandard boolean validity checking

[Ada] Optimize nonstandard boolean validity checking

Validity checking of enumerations with nonstandard representation
starts by checking the value range, then calling _rep_to_pos to verify
that the value itself is valid. The value range check is thus
redundant and inefficient: the _rep_to_pos call is normally inlined
when optimizing for speed and the range check slows down the fast
path; it is unnecesary and undesirable when optimizing for size, and
just unnecessary when not optimizing. This patch thus drops the range
check for nonstandard boolean types.

gcc/ada/

	* exp_attr.adb (Expand_N_Attribute_Reference) <Attribute_Valid>:
	Drop redundant range check for nonstandard booleans.