Use simplify_gen_binary in canon_rtx

After simplifying the operands of a PLUS, canon_rtx checked only
for cases in which one of the simplified operands was a constant,
falling back to gen_rtx_PLUS otherwise.  This left the PLUS in a
non-canonical order if one of the simplified operands was
(plus (reg R1) (const_int X)); we'd end up with:

       (plus (plus (reg R1) (const_int Y)) (reg R2))

rather than:

       (plus (plus (reg R1) (reg R2)) (const_int Y))

Fixing this exposed new DSE opportunities on spu-elf in
gcc.c-torture/execute/builtins/strcat-chk.c but otherwise
it doesn't seem to have much practical effect.

gcc/
2016-11-15  Richard Sandiford  <richard.sandiford@arm.com>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

	* alias.c (canon_rtx): Use simplify_gen_binary.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>

From-SVN: r242445

gcc/ChangeLog

@@ -2,6 +2,12 @@
 	    Alan Hayward  <alan.hayward@arm.com>
 	    David Sherwood  <david.sherwood@arm.com>
 
+	* alias.c (canon_rtx): Use simplify_gen_binary.
+
+2016-11-15  Richard Sandiford  <richard.sandiford@arm.com>
+	    Alan Hayward  <alan.hayward@arm.com>
+	    David Sherwood  <david.sherwood@arm.com>
+
 	* rtl.h (load_extend_op): Declare.
 	* rtlanal.c (load_extend_op): New function.
 	(nonzero_bits1): Use it.

gcc/alias.c

@@ -1708,13 +1708,7 @@ canon_rtx (rtx x)
       rtx x1 = canon_rtx (XEXP (x, 1));
 
       if (x0 != XEXP (x, 0) || x1 != XEXP (x, 1))
-	{
-	  if (CONST_INT_P (x0))
-	    return plus_constant (GET_MODE (x), x1, INTVAL (x0));
-	  else if (CONST_INT_P (x1))
-	    return plus_constant (GET_MODE (x), x0, INTVAL (x1));
-	  return gen_rtx_PLUS (GET_MODE (x), x0, x1);
-	}
+	return simplify_gen_binary (PLUS, GET_MODE (x), x0, x1);
     }
 
   /* This gives us much better alias analysis when called from