[gomp4] use integer_zerop

[Nathan Sidwell]

I've applied this, where I'd incorrectly compared nodes for equality, rather 
than the node value.

nathan
2015-09-09  Nathan Sidwell  <nat...@codesourcery.com>
	* omp-low.c (execute_oacc_transform): Use integer_zerop, not
	direct node comparison.

Index: gcc/omp-low.c
===================================================================
--- gcc/omp-low.c	(revision 227599)
+++ gcc/omp-low.c	(working copy)
@@ -14829,7 +14829,7 @@ execute_oacc_transform ()
 
 	if (purpose[ix])
 	  {
-	    if (purpose[ix] == boolean_false_node)
+	    if (integer_zerop (purpose[ix]))
 	      fn_level = ix + 1;
 	    else if (fn_level < 0)
 	      fn_level = ix;