coroutines: Fix STRIP_NOPS usage.

parm = STRIP_NOPS (parm); is unnecessary and generates
warning: operation on 'parm' may be undefined [-Wsequence-point]
when cp/coroutines.cc is compiled with -std=c++11.

	* coroutines.cc (captures_temporary): Don't assign the result of
	STRIP_NOPS to the same variable.