gimplify_modify_expr: avoid DECL_DEBUG_EXPR links across functions

An upcoming patch exposes a bug in gimplify_modify_expr.  There, we try
not to create DECL_DEBUG_EXPR links across functions, however we don't
check that *FROM_P actually belongs to the current function before
modifying it.  This patch fixes this oversight.

gcc/

	* gimplify.c (gimplify_modify_expr): Don't create a
	DECL_DEBUG_EXPR link if *FROM_P does not belong to the current
	function.

From-SVN: r248688

gcc/ChangeLog

+2017-05-30  Pierre-Marie de Rodat  <derodat@adacore.com>
+
+	* gimplify.c (gimplify_modify_expr): Don't create a
+	DECL_DEBUG_EXPR link if *FROM_P does not belong to the current
+	function.
+
 2017-05-30  Wilco Dijkstra  <wdijkstr@arm.com>
 
 	* config/arm/arm-builtins.c (arm_expand_builtin): Remove const.

gcc/gimplify.c

@@ -5580,7 +5580,8 @@ gimplify_modify_expr (tree *expr_p, gimple_seq *pre_p, gimple_seq *post_p,
       && DECL_IGNORED_P (*from_p)
       && DECL_P (*to_p)
       && !DECL_IGNORED_P (*to_p)
-      && decl_function_context (*to_p) == current_function_decl)
+      && decl_function_context (*to_p) == current_function_decl
+      && decl_function_context (*from_p) == current_function_decl)
     {
       if (!DECL_NAME (*from_p) && DECL_NAME (*to_p))
 	DECL_NAME (*from_p)