coroutines : Avoid generating empty statements [PR96749].

coroutines : Avoid generating empty statements [PR96749].

In the compiler-only idiom:
" a = (target expr creats temp, op uses temp) "
the target expression variable needs to be promoted to a frame one
(if the expression has a suspend point).  However, the only uses of
the var are in the second part of the compound expression - and we
were creating an empty statement corresponding to the (now unused)
first arm.  This then produces the spurious warnings noted.

Fixed by avoiding generation of a separate variable nest for
isolated target expressions (or similarly isolated co_awaits used
in a function call).

gcc/cp/ChangeLog:

	PR c++/96749
	* coroutines.cc (flatten_await_stmt): Allow for the case
	where a target expression variable only has uses in the
	second part of a compound expression.
	(maybe_promote_temps): Avoid emiting empty statements.

gcc/testsuite/ChangeLog:

	PR c++/96749
	* g++.dg/coroutines/pr96749-1.C: New test.
	* g++.dg/coroutines/pr96749-2.C: New test.