coroutines: Small cleanups to await_statement_walker [NFC].

There is no need to make a MODIFY_EXPR for any of the condition
vars that we synthesize.

Expansion of co_return can be carried out independently of any
co_awaits that might be contained which simplifies this.

Where we are rewriting statements to handle await expression
logic, there is no need to carry out any analysis - we just need
to detect the presence of any co_await.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>

gcc/cp/ChangeLog:

	* coroutines.cc (await_statement_walker): Code cleanups.