[Ada] Cleanup work in internal support for renaming (1)

[Arnaud Charlet <charlet at adacore dot com>]

In Ada, the result of evaluating a function call is a constant object and,
therefore, can be subject to a renaming.  This is fully implemented in the
compiler by creating a temporary to capture the return value, either in gigi
for simple cases or in the front-end for more convoluted cases.

The front-end can also generate internal renamings, typically to remove the
side effects of an expression before manipulating it.  This is more efficient
than generating a temporary when the renamed object is really a name.

But generating internal renamings of function calls is to be avoided because
they will inevitably be converted into a temporary at some point, so it is
both conceptually simpler and more efficient to generate temporaries directly.

Remove_Side_Effects already knows to avoid renaming function calls in most
cases and this change deals with two more sources of these renamings.

No functional changes.

Tested on x86_64-pc-linux-gnu, committed on trunk

2015-05-26  Eric Botcazou  <ebotcazou@adacore.com>

	* exp_ch6.adb (Expand_Actuals): Use a constant declaration instead
	of a renaming to capture the return value of a function call.
	(Expand_Simple_Function_Return): Call Remove_Side_Effects
	instead of removing side effects manually before the call to
	_Postconditions.

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