[Ada] Single-element Append performance improvement

[Ada] Single-element Append performance improvement

Ada.Containers.Vectors has two Append procedures that take an
Element value; one takes a Count parameter and one does not
(the count is implicitly one for the latter). For the former version,
there was code that took a faster path if certain conditions were met
and otherwise took a slower path; one of the prerequisite conditions
for this was Count = 1. For the latter version, no such special-case
detection was performed; the more general code was always executed.
Move the special-case detection/handling code from the former version into
the latter and change the former version to simply call the latter version
if Count = 1. Also apply same change to Ada.Containers.Indefinite_Vectors.

gcc/ada/

	* libgnat/a-coinve.adb, libgnat/a-convec.adb
	(Append): If the Append that takes an Element and a Count is
	called with Count = 1, then call the Append that does not take a
	Count parameter; otherwise call the code that handles the general
	case. Move the special case detection/handling code that was
	formerly in that version of Append into the version that does not
	take a Count parameter, so that now both versions get the
	performance benefit.