Prevent use of MEM_* attr accessor macros as lvalues

gcc/ChangeLog:
	* rtl.h (get_mem_attrs): Add "const" qualifier to returned
	pointer.

From-SVN: r243487

gcc/ChangeLog

+2016-12-09  David Malcolm  <dmalcolm@redhat.com>
+
+	* rtl.h (get_mem_attrs): Add "const" qualifier to returned
+	pointer.
+
 2016-12-09  Nathan Sidwell  <nathan@acm.org>
 
 	PR C++/78550

gcc/rtl.h

@@ -3355,7 +3355,7 @@ extern struct target_rtl *this_target_rtl;
 
 #ifndef GENERATOR_FILE
 /* Return the attributes of a MEM rtx.  */
-static inline struct mem_attrs *
+static inline const struct mem_attrs *
 get_mem_attrs (const_rtx x)
 {
   struct mem_attrs *attrs;