[AArch64] Reject (high (const (plus anchor offset)))

The aarch64_legitimate_constant_p tests for HIGH and CONST seem
to be the wrong way round: (high (const ...)) is valid rtl that
could be passed in, but (const (high ...)) isn't.  As it stands,
we disallow anchor+offset but allow (high anchor+offset).

2018-01-09  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* config/aarch64/aarch64.c (aarch64_legitimate_constant_p): Fix
	order of HIGH and CONST checks.

From-SVN: r256384

gcc/ChangeLog

 2018-01-09  Richard Sandiford  <richard.sandiford@linaro.org>
 
+	* config/aarch64/aarch64.c (aarch64_legitimate_constant_p): Fix
+	order of HIGH and CONST checks.
+
+2018-01-09  Richard Sandiford  <richard.sandiford@linaro.org>
+
 	* tree-vect-stmts.c (permute_vec_elements): Create a fresh variable
 	if the destination isn't an SSA_NAME.
 

gcc/config/aarch64/aarch64.c

@@ -10449,6 +10449,9 @@ aarch64_legitimate_constant_p (machine_mode mode, rtx x)
   if (CONST_WIDE_INT_P (x))
     return false;
 
+  if (GET_CODE (x) == HIGH)
+    x = XEXP (x, 0);
+
   /* Do not allow const (plus (anchor_symbol, const_int)).  */
   if (GET_CODE (x) == CONST)
     {
@@ -10460,9 +10463,6 @@ aarch64_legitimate_constant_p (machine_mode mode, rtx x)
 	return false;
     }
 
-  if (GET_CODE (x) == HIGH)
-    x = XEXP (x, 0);
-
   /* Treat symbols as constants.  Avoid TLS symbols as they are complex,
      so spilling them is better than rematerialization.  */
   if (SYMBOL_REF_P (x) && !SYMBOL_REF_TLS_MODEL (x))