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.

gcc/cp/ChangeLog

+2020-04-20  Marek Polacek  <polacek@redhat.com>
+
+	* coroutines.cc (captures_temporary): Don't assign the result of
+	STRIP_NOPS to the same variable.
+
 2020-04-20  Nathan Sidwell  <nathan@acm.org>
 
 	PR 94454 - tpl-tpl-parms are not canonicalizable types

gcc/cp/coroutines.cc

@@ -2652,7 +2652,7 @@ captures_temporary (tree *stmt, int *do_subtree, void *d)
 	  parm = TREE_OPERAND (parm, 0);
 	  if (TREE_CODE (parm) == INDIRECT_REF)
 	    parm = TREE_OPERAND (parm, 0);
-	  parm = STRIP_NOPS (parm);
+	  STRIP_NOPS (parm);
 	}
 
       /* This isn't a temporary.  */