ada: Make detection of useless copy for return more robust

ada: Make detection of useless copy for return more robust

In the return-by-invisible-reference case, the return object of an extended
return statement is allocated directly on the return stack and, therefore,
the copy operation on return is useless.  The code detecting this was not
robust enough and missed some complex cases.

gcc/ada/

	* gcc-interface/trans.cc (gnat_to_gnu) <N_Simple_Return_Statement>:
	In the return-by-invisible-reference case, remove conversions before
	looking for a dereference in the return values and building the test
	protecting against a useless copy operation.