Minor vn_reference_lookup_3 tweak

The repeated checks for MEM_REF made this code hard to convert to
poly_ints as-is.  Hopefully the new structure also makes it clearer
at a glance what the two cases are.

2017-12-16  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* tree-ssa-sccvn.c (vn_reference_lookup_3): Avoid repeated
	checks for MEM_REF.

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

From-SVN: r255742

gcc/ChangeLog

@@ -2,6 +2,13 @@
 	    Alan Hayward  <alan.hayward@arm.com>
 	    David Sherwood  <david.sherwood@arm.com>
 
+	* tree-ssa-sccvn.c (vn_reference_lookup_3): Avoid repeated
+	checks for MEM_REF.
+
+2017-12-16  Richard Sandiford  <richard.sandiford@linaro.org>
+	    Alan Hayward  <alan.hayward@arm.com>
+	    David Sherwood  <david.sherwood@arm.com>
+
 	* doc/generic.texi (VEC_SERIES_EXPR): Document.
 	* doc/md.texi (vec_series@var{m}): Document.
 	* tree.def (VEC_SERIES_EXPR): New tree code.

gcc/tree-ssa-sccvn.c

@@ -2348,19 +2348,19 @@ vn_reference_lookup_3 (ao_ref *ref, tree vuse, void *vr_,
       copy_size = tree_to_uhwi (gimple_call_arg (def_stmt, 2));
 
       /* The bases of the destination and the references have to agree.  */
-      if ((TREE_CODE (base) != MEM_REF
-	   && !DECL_P (base))
-	  || (TREE_CODE (base) == MEM_REF
-	      && (TREE_OPERAND (base, 0) != lhs
-		  || !tree_fits_uhwi_p (TREE_OPERAND (base, 1))))
-	  || (DECL_P (base)
-	      && (TREE_CODE (lhs) != ADDR_EXPR
-		  || TREE_OPERAND (lhs, 0) != base)))
-	return (void *)-1;
-
       at = offset / BITS_PER_UNIT;
       if (TREE_CODE (base) == MEM_REF)
-	at += tree_to_uhwi (TREE_OPERAND (base, 1));
+	{
+	  if (TREE_OPERAND (base, 0) != lhs
+	      || !tree_fits_uhwi_p (TREE_OPERAND (base, 1)))
+	    return (void *) -1;
+	  at += tree_to_uhwi (TREE_OPERAND (base, 1));
+	}
+      else if (!DECL_P (base)
+	       || TREE_CODE (lhs) != ADDR_EXPR
+	       || TREE_OPERAND (lhs, 0) != base)
+	return (void *)-1;
+
       /* If the access is completely outside of the memcpy destination
 	 area there is no aliasing.  */
       if (lhs_offset >= at + maxsize / BITS_PER_UNIT