arm: Fix bootstrap failure with rtl-checking

The code change that caused this regression was not meant to affect neon
code-gen, however I missed the REG fall through.  This patch makes sure we only
get the left-hand of the PLUS if it is indeed a PLUS expr.

gcc/ChangeLog:
2020-04-27  Andre Vieira  <andre.simoesdiasvieira@arm.com>

	* config/arm/arm.c (output_move_neon): Only get the first operand if
	addr is PLUS.

gcc/ChangeLog

+2020-04-27  Andre Vieira  <andre.simoesdiasvieira@arm.com>
+
+	* config/arm/arm.c (output_move_neon): Only get the first operand if
+	addr is PLUS.
+
 2020-04-27  Felix Yang  <felix.yang@huawei.com>
 
 	PR tree-optimization/94784

gcc/config/arm/arm.c

@@ -20145,7 +20145,8 @@ output_move_neon (rtx *operands)
 	}
       /* Fall through.  */
     case PLUS:
-      addr = XEXP (addr, 0);
+      if (GET_CODE (addr) == PLUS)
+	addr = XEXP (addr, 0);
       /* Fall through.  */
     case LABEL_REF:
       {